SELECT, TABLE, WITH — retrieve rows from a table or view
[ WITH [ RECURSIVE ]with_query[, ...] ] SELECT [ ALL | DISTINCT [ ON (expression[, ...] ) ] ] [ { * |expression[ [ AS ]output_name] } [, ...] ] [ FROMfrom_item[, ...] ] [ WHEREcondition] [ GROUP BY [ ALL | DISTINCT ]grouping_element[, ...] ] [ HAVINGcondition] [ WINDOWwindow_nameAS (window_definition) [, ...] ] [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ]select] [ ORDER BYexpression[ ASC | DESC | USINGoperator] [ NULLS { FIRST | LAST } ] [, ...] ] [ LIMIT {count| ALL } ] [ OFFSETstart[ ROW | ROWS ] ] [ FETCH { FIRST | NEXT } [count] { ROW | ROWS } { ONLY | WITH TIES } ] [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OFtable_name[, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ] wherefrom_itemcan be one of: [ ONLY ]table_name[ * ] [ [ AS ]alias[ (column_alias[, ...] ) ] ] [ TABLESAMPLEsampling_method(argument[, ...] ) [ REPEATABLE (seed) ] ] [ LATERAL ] (select) [ AS ]alias[ (column_alias[, ...] ) ]with_query_name[ [ AS ]alias[ (column_alias[, ...] ) ] ] [ LATERAL ]function_name( [argument[, ...] ] ) [ WITH ORDINALITY ] [ [ AS ]alias[ (column_alias[, ...] ) ] ] [ LATERAL ]function_name( [argument[, ...] ] ) [ AS ]alias(column_definition[, ...] ) [ LATERAL ]function_name( [argument[, ...] ] ) AS (column_definition[, ...] ) [ LATERAL ] ROWS FROM(function_name( [argument[, ...] ] ) [ AS (column_definition[, ...] ) ] [, ...] ) [ WITH ORDINALITY ] [ [ AS ]alias[ (column_alias[, ...] ) ] ]from_itemjoin_typefrom_item{ ONjoin_condition| USING (join_column[, ...] ) [ ASjoin_using_alias] }from_itemNATURALjoin_typefrom_itemfrom_itemCROSS JOINfrom_itemandgrouping_elementcan be one of: ( )expression(expression[, ...] ) ROLLUP ( {expression| (expression[, ...] ) } [, ...] ) CUBE ( {expression| (expression[, ...] ) } [, ...] ) GROUPING SETS (grouping_element[, ...] ) andwith_queryis:with_query_name[ (column_name[, ...] ) ] AS [ [ NOT ] MATERIALIZED ] (select|values|insert|update|delete) [ SEARCH { BREADTH | DEPTH } FIRST BYcolumn_name[, ...] SETsearch_seq_col_name] [ CYCLEcolumn_name[, ...] SETcycle_mark_col_name[ TOcycle_mark_valueDEFAULTcycle_mark_default] USINGcycle_path_col_name] TABLE [ ONLY ]table_name[ * ]
   SELECT retrieves rows from zero or more tables.
   The general processing of SELECT is as follows:
   
      All queries in the WITH list are computed.
      These effectively serve as temporary tables that can be referenced
      in the FROM list.  A WITH query
      that is referenced more than once in FROM is
      computed only once,
      unless specified otherwise with NOT MATERIALIZED.
      (See WITH Clause below.)
     
      All elements in the FROM list are computed.
      (Each element in the FROM list is a real or
      virtual table.)  If more than one element is specified in the
      FROM list, they are cross-joined together.
      (See FROM Clause below.)
     
      If the WHERE clause is specified, all rows
      that do not satisfy the condition are eliminated from the
      output.  (See WHERE Clause below.)
     
      If the GROUP BY clause is specified,
      or if there are aggregate function calls, the
      output is combined into groups of rows that match on one or more
      values, and the results of aggregate functions are computed.
      If the HAVING clause is present, it
      eliminates groups that do not satisfy the given condition.  (See
      GROUP BY Clause and
      HAVING Clause below.)
      Although query output columns are nominally computed in the next
      step, they can also be referenced (by name or ordinal number)
      in the GROUP BY clause.
     
      The actual output rows are computed using the
      SELECT output expressions for each selected
      row or row group.  (See SELECT List below.)
     
SELECT DISTINCT eliminates duplicate rows from the
      result.  SELECT DISTINCT ON eliminates rows that
      match on all the specified expressions.  SELECT ALL
      (the default) will return all candidate rows, including
      duplicates.  (See DISTINCT Clause below.)
     
      Using the operators UNION,
      INTERSECT, and EXCEPT, the
      output of more than one SELECT statement can
      be combined to form a single result set.  The
      UNION operator returns all rows that are in
      one or both of the result sets.  The
      INTERSECT operator returns all rows that are
      strictly in both result sets.  The EXCEPT
      operator returns the rows that are in the first result set but
      not in the second.  In all three cases, duplicate rows are
      eliminated unless ALL is specified.  The noise
      word DISTINCT can be added to explicitly specify
      eliminating duplicate rows.  Notice that DISTINCT is
      the default behavior here, even though ALL is
      the default for SELECT itself.  (See
      UNION Clause, INTERSECT Clause, and
      EXCEPT Clause below.)
     
      If the ORDER BY clause is specified, the
      returned rows are sorted in the specified order.  If
      ORDER BY is not given, the rows are returned
      in whatever order the system finds fastest to produce.  (See
      ORDER BY Clause below.)
     
      If the LIMIT (or FETCH FIRST) or OFFSET
      clause is specified, the SELECT statement
      only returns a subset of the result rows. (See LIMIT Clause below.)
     
      If FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE
      or FOR KEY SHARE
      is specified, the
      SELECT statement locks the selected rows
      against concurrent updates.  (See The Locking Clause
      below.)
     
   You must have SELECT privilege on each column used
   in a SELECT command.  The use of FOR NO KEY UPDATE,
   FOR UPDATE,
   FOR SHARE or FOR KEY SHARE requires
   UPDATE privilege as well (for at least one column
   of each table so selected).
  
WITH Clause
    The WITH clause allows you to specify one or more
    subqueries that can be referenced by name in the primary query.
    The subqueries effectively act as temporary tables or views
    for the duration of the primary query.
    Each subquery can be a SELECT, TABLE, VALUES,
    INSERT, UPDATE or
    DELETE statement.
    When writing a data-modifying statement (INSERT,
    UPDATE or DELETE) in
    WITH, it is usual to include a RETURNING clause.
    It is the output of RETURNING, not the underlying
    table that the statement modifies, that forms the temporary table that is
    read by the primary query.  If RETURNING is omitted, the
    statement is still executed, but it produces no output so it cannot be
    referenced as a table by the primary query.
   
    A name (without schema qualification) must be specified for each
    WITH query.  Optionally, a list of column names
    can be specified; if this is omitted,
    the column names are inferred from the subquery.
   
    If RECURSIVE is specified, it allows a
    SELECT subquery to reference itself by name.  Such a
    subquery must have the form
non_recursive_termUNION [ ALL | DISTINCT ]recursive_term
    where the recursive self-reference must appear on the right-hand
    side of the UNION.  Only one recursive self-reference
    is permitted per query.  Recursive data-modifying statements are not
    supported, but you can use the results of a recursive
    SELECT query in
    a data-modifying statement.  See Section 7.8 for
    an example.
   
    Another effect of RECURSIVE is that
    WITH queries need not be ordered: a query
    can reference another one that is later in the list.  (However,
    circular references, or mutual recursion, are not implemented.)
    Without RECURSIVE, WITH queries
    can only reference sibling WITH queries
    that are earlier in the WITH list.
   
    When there are multiple queries in the WITH
    clause, RECURSIVE should be written only once,
    immediately after WITH.  It applies to all queries
    in the WITH clause, though it has no effect on
    queries that do not use recursion or forward references.
   
    The optional SEARCH clause computes a search
    sequence column that can be used for ordering the results of a
    recursive query in either breadth-first or depth-first order.  The
    supplied column name list specifies the row key that is to be used for
    keeping track of visited rows.  A column named
    search_seq_col_name will be added to the result
    column list of the WITH query.  This column can be
    ordered by in the outer query to achieve the respective ordering.  See
    Section 7.8.2.1 for examples.
   
    The optional CYCLE clause is used to detect cycles in
    recursive queries.  The supplied column name list specifies the row key
    that is to be used for keeping track of visited rows.  A column named
    cycle_mark_col_name will be added to the result
    column list of the WITH query.  This column will be set
    to cycle_mark_value when a cycle has been
    detected, else to cycle_mark_default.
    Furthermore, processing of the recursive union will stop when a cycle has
    been detected.  cycle_mark_value and
    cycle_mark_default must be constants and they
    must be coercible to a common data type, and the data type must have an
    inequality operator.  (The SQL standard requires that they be Boolean
    constants or character strings, but PostgreSQL does not require that.)  By
    default, TRUE and FALSE (of type
    boolean) are used.  Furthermore, a column
    named cycle_path_col_name will be added to the
    result column list of the WITH query.  This column is
    used internally for tracking visited rows.  See Section 7.8.2.2 for examples.
   
    Both the SEARCH and the CYCLE clause
    are only valid for recursive WITH queries.  The
    with_query must be a UNION
    (or UNION ALL) of two SELECT (or
    equivalent) commands (no nested UNIONs).  If both
    clauses are used, the column added by the SEARCH clause
    appears before the columns added by the CYCLE clause.
   
    The primary query and the WITH queries are all
    (notionally) executed at the same time.  This implies that the effects of
    a data-modifying statement in WITH cannot be seen from
    other parts of the query, other than by reading its RETURNING
    output.  If two such data-modifying statements attempt to modify the same
    row, the results are unspecified.
   
    A key property of WITH queries is that they
    are normally evaluated only once per execution of the primary query,
    even if the primary query refers to them more than once.
    In particular, data-modifying statements are guaranteed to be
    executed once and only once, regardless of whether the primary query
    reads all or any of their output.
   
    However, a WITH query can be marked
    NOT MATERIALIZED to remove this guarantee.  In that
    case, the WITH query can be folded into the primary
    query much as though it were a simple sub-SELECT in
    the primary query's FROM clause.  This results in
    duplicate computations if the primary query refers to
    that WITH query more than once; but if each such use
    requires only a few rows of the WITH query's total
    output, NOT MATERIALIZED can provide a net savings by
    allowing the queries to be optimized jointly.
    NOT MATERIALIZED is ignored if it is attached to
    a WITH query that is recursive or is not
    side-effect-free (i.e., is not a plain SELECT
    containing no volatile functions).
   
    By default, a side-effect-free WITH query is folded
    into the primary query if it is used exactly once in the primary
    query's FROM clause.  This allows joint optimization
    of the two query levels in situations where that should be semantically
    invisible.  However, such folding can be prevented by marking the
    WITH query as MATERIALIZED.
    That might be useful, for example, if the WITH query
    is being used as an optimization fence to prevent the planner from
    choosing a bad plan.
    PostgreSQL versions before v12 never did
    such folding, so queries written for older versions might rely on
    WITH to act as an optimization fence.
   
See Section 7.8 for additional information.
FROM Clause
    The FROM clause specifies one or more source
    tables for the SELECT.  If multiple sources are
    specified, the result is the Cartesian product (cross join) of all
    the sources.  But usually qualification conditions are added (via
    WHERE) to restrict the returned rows to a small subset of the
    Cartesian product.
   
    The FROM clause can contain the following
    elements:
    
table_name
        The name (optionally schema-qualified) of an existing table or view.
        If ONLY is specified before the table name, only that
        table is scanned.  If ONLY is not specified, the table
        and all its descendant tables (if any) are scanned.  Optionally,
        * can be specified after the table name to explicitly
        indicate that descendant tables are included.
       
alias
        A substitute name for the FROM item containing the
        alias.  An alias is used for brevity or to eliminate ambiguity
        for self-joins (where the same table is scanned multiple
        times).  When an alias is provided, it completely hides the
        actual name of the table or function; for example given
        FROM foo AS f, the remainder of the
        SELECT must refer to this FROM
        item as f not foo.  If an alias is
        written, a column alias list can also be written to provide
        substitute names for one or more columns of the table.
       
TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ]
        A TABLESAMPLE clause after
        a table_name indicates that the
        specified sampling_method
        should be used to retrieve a subset of the rows in that table.
        This sampling precedes the application of any other filters such
        as WHERE clauses.
        The standard PostgreSQL distribution
        includes two sampling methods, BERNOULLI
        and SYSTEM, and other sampling methods can be
        installed in the database via extensions.
       
        The BERNOULLI and SYSTEM sampling methods
        each accept a single argument
        which is the fraction of the table to sample, expressed as a
        percentage between 0 and 100.  This argument can be
        any real-valued expression.  (Other sampling methods might
        accept more or different arguments.)  These two methods each return
        a randomly-chosen sample of the table that will contain
        approximately the specified percentage of the table's rows.
        The BERNOULLI method scans the whole table and
        selects or ignores individual rows independently with the specified
        probability.
        The SYSTEM method does block-level sampling with
        each block having the specified chance of being selected; all rows
        in each selected block are returned.
        The SYSTEM method is significantly faster than
        the BERNOULLI method when small sampling
        percentages are specified, but it may return a less-random sample of
        the table as a result of clustering effects.
       
        The optional REPEATABLE clause specifies
        a seed number or expression to use
        for generating random numbers within the sampling method.  The seed
        value can be any non-null floating-point value.  Two queries that
        specify the same seed and argument
        values will select the same sample of the table, if the table has
        not been changed meanwhile.  But different seed values will usually
        produce different samples.
        If REPEATABLE is not given then a new random
        sample is selected for each query, based upon a system-generated seed.
        Note that some add-on sampling methods do not
        accept REPEATABLE, and will always produce new
        samples on each use.
       
select
        A sub-SELECT can appear in the
        FROM clause.  This acts as though its
        output were created as a temporary table for the duration of
        this single SELECT command.  Note that the
        sub-SELECT must be surrounded by
        parentheses, and an alias must be
        provided for it.  A
        VALUES command
        can also be used here.
       
with_query_name
        A WITH query is referenced by writing its name,
        just as though the query's name were a table name.  (In fact,
        the WITH query hides any real table of the same name
        for the purposes of the primary query.  If necessary, you can
        refer to a real table of the same name by schema-qualifying
        the table's name.)
        An alias can be provided in the same way as for a table.
       
function_name
        Function calls can appear in the FROM
        clause.  (This is especially useful for functions that return
        result sets, but any function can be used.)  This acts as
        though the function's output were created as a temporary table for the
        duration of this single SELECT command.
        If the function's result type is composite (including the case of a
        function with multiple OUT parameters), each
        attribute becomes a separate column in the implicit table.
       
        When the optional WITH ORDINALITY clause is added
        to the function call, an additional column of type bigint
        will be appended to the function's result column(s).  This column
        numbers the rows of the function's result set, starting from 1.
        By default, this column is named ordinality.
       
An alias can be provided in the same way as for a table. If an alias is written, a column alias list can also be written to provide substitute names for one or more attributes of the function's composite return type, including the ordinality column if present.
        Multiple function calls can be combined into a
        single FROM-clause item by surrounding them
        with ROWS FROM( ... ).  The output of such an item is the
        concatenation of the first row from each function, then the second
        row from each function, etc.  If some of the functions produce fewer
        rows than others, null values are substituted for the missing data, so
        that the total number of rows returned is always the same as for the
        function that produced the most rows.
       
        If the function has been defined as returning the
        record data type, then an alias or the key word
        AS must be present, followed by a column
        definition list in the form ( .  The column definition list must match the
        actual number and types of columns returned by the function.
       column_name data_type [, ...
        ])
        When using the ROWS FROM( ... ) syntax, if one of the
        functions requires a column definition list, it's preferred to put
        the column definition list after the function call inside
        ROWS FROM( ... ).  A column definition list can be placed
        after the ROWS FROM( ... ) construct only if there's just
        a single function and no WITH ORDINALITY clause.
       
        To use ORDINALITY together with a column definition
        list, you must use the ROWS FROM( ... ) syntax and put the
        column definition list inside ROWS FROM( ... ).
       
join_typeOne of
[ INNER ] JOIN
LEFT [ OUTER ] JOIN
RIGHT [ OUTER ] JOIN
FULL [ OUTER ] JOIN
        For the INNER and OUTER join types, a
        join condition must be specified, namely exactly one of
        ON ,
        join_conditionUSING (,
        or join_column [, ...])NATURAL.  See below for the meaning.
       
        A JOIN clause combines two FROM
        items, which for convenience we will refer to as “tables”,
        though in reality they can be any type of FROM item.
        Use parentheses if necessary to determine the order of nesting.
        In the absence of parentheses, JOINs nest
        left-to-right.  In any case JOIN binds more
        tightly than the commas separating FROM-list items.
        All the JOIN options are just a notational
        convenience, since they do nothing you couldn't do with plain
        FROM and WHERE.
       
LEFT OUTER JOIN returns all rows in the qualified
        Cartesian product (i.e., all combined rows that pass its join
        condition), plus one copy of each row in the left-hand table
        for which there was no right-hand row that passed the join
        condition.  This left-hand row is extended to the full width
        of the joined table by inserting null values for the
        right-hand columns.  Note that only the JOIN
        clause's own condition is considered while deciding which rows
        have matches.  Outer conditions are applied afterwards.
       
        Conversely, RIGHT OUTER JOIN returns all the
        joined rows, plus one row for each unmatched right-hand row
        (extended with nulls on the left).  This is just a notational
        convenience, since you could convert it to a LEFT
        OUTER JOIN by switching the left and right tables.
       
FULL OUTER JOIN returns all the joined rows, plus
        one row for each unmatched left-hand row (extended with nulls
        on the right), plus one row for each unmatched right-hand row
        (extended with nulls on the left).
       
ON join_conditionjoin_condition is
        an expression resulting in a value of type
        boolean (similar to a WHERE
        clause) that specifies which rows in a join are considered to
        match.
       
USING ( join_column [, ...] ) [ AS join_using_alias ]
        A clause of the form USING ( a, b, ... ) is
        shorthand for ON left_table.a = right_table.a AND
        left_table.b = right_table.b ....  Also,
        USING implies that only one of each pair of
        equivalent columns will be included in the join output, not
        both.
       
        If a join_using_alias
        name is specified, it provides a table alias for the join columns.
        Only the join columns listed in the USING clause
        are addressable by this name.  Unlike a regular alias, this does not hide the names of
        the joined tables from the rest of the query.  Also unlike a regular
        alias, you cannot write a
        column alias list — the output names of the join columns are the
        same as they appear in the USING list.
       
NATURAL
        NATURAL is shorthand for a
        USING list that mentions all columns in the two
        tables that have matching names.  If there are no common
        column names, NATURAL is equivalent
        to ON TRUE.
       
CROSS JOIN
        CROSS JOIN is equivalent to INNER JOIN ON
        (TRUE), that is, no rows are removed by qualification.
        They produce a simple Cartesian product, the same result as you get from
        listing the two tables at the top level of FROM,
        but restricted by the join condition (if any).
       
LATERAL
        The LATERAL key word can precede a
        sub-SELECT FROM item.  This allows the
        sub-SELECT to refer to columns of FROM
        items that appear before it in the FROM list.  (Without
        LATERAL, each sub-SELECT is
        evaluated independently and so cannot cross-reference any other
        FROM item.)
       
LATERAL can also precede a function-call
        FROM item, but in this case it is a noise word, because
        the function expression can refer to earlier FROM items
        in any case.
       
        A LATERAL item can appear at top level in the
        FROM list, or within a JOIN tree.  In the
        latter case it can also refer to any items that are on the left-hand
        side of a JOIN that it is on the right-hand side of.
       
        When a FROM item contains LATERAL
        cross-references, evaluation proceeds as follows: for each row of the
        FROM item providing the cross-referenced column(s), or
        set of rows of multiple FROM items providing the
        columns, the LATERAL item is evaluated using that
        row or row set's values of the columns.  The resulting row(s) are
        joined as usual with the rows they were computed from.  This is
        repeated for each row or set of rows from the column source table(s).
       
        The column source table(s) must be INNER or
        LEFT joined to the LATERAL item, else
        there would not be a well-defined set of rows from which to compute
        each set of rows for the LATERAL item.  Thus,
        although a construct such as X RIGHT JOIN
        LATERAL YY to reference
        X.
       
WHERE Clause
    The optional WHERE clause has the general form
WHERE condition
    where condition is
    any expression that evaluates to a result of type
    boolean.  Any row that does not satisfy this
    condition will be eliminated from the output.  A row satisfies the
    condition if it returns true when the actual row values are
    substituted for any variable references.
   
GROUP BY Clause
    The optional GROUP BY clause has the general form
GROUP BY [ ALL | DISTINCT ] grouping_element [, ...]
    GROUP BY will condense into a single row all
    selected rows that share the same values for the grouped
    expressions.  An expression used inside a
    grouping_element
    can be an input column name, or the name or ordinal number of an
    output column (SELECT list item), or an arbitrary
    expression formed from input-column values.  In case of ambiguity,
    a GROUP BY name will be interpreted as an
    input-column name rather than an output column name.
   
    If any of GROUPING SETS, ROLLUP or
    CUBE are present as grouping elements, then the
    GROUP BY clause as a whole defines some number of
    independent grouping sets.  The effect of this is
    equivalent to constructing a UNION ALL between
    subqueries with the individual grouping sets as their
    GROUP BY clauses.  The optional DISTINCT
    clause removes duplicate sets before processing; it does not
    transform the UNION ALL into a UNION DISTINCT.
    For further details on the handling
    of grouping sets see Section 7.2.4.
   
    Aggregate functions, if any are used, are computed across all rows
    making up each group, producing a separate value for each group.
    (If there are aggregate functions but no GROUP BY
    clause, the query is treated as having a single group comprising all
    the selected rows.)
    The set of rows fed to each aggregate function can be further filtered by
    attaching a FILTER clause to the aggregate function
    call; see Section 4.2.7 for more information.  When
    a FILTER clause is present, only those rows matching it
    are included in the input to that aggregate function.
   
    When GROUP BY is present,
    or any aggregate functions are present, it is not valid for
    the SELECT list expressions to refer to
    ungrouped columns except within aggregate functions or when the
    ungrouped column is functionally dependent on the grouped columns,
    since there would otherwise be more than one possible value to
    return for an ungrouped column.  A functional dependency exists if
    the grouped columns (or a subset thereof) are the primary key of
    the table containing the ungrouped column.
   
    Keep in mind that all aggregate functions are evaluated before
    evaluating any “scalar” expressions in the HAVING
    clause or SELECT list.  This means that, for example,
    a CASE expression cannot be used to skip evaluation of
    an aggregate function; see Section 4.2.14.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE and FOR KEY SHARE cannot be
    specified with GROUP BY.
   
HAVING Clause
    The optional HAVING clause has the general form
HAVING condition
    where condition is
    the same as specified for the WHERE clause.
   
    HAVING eliminates group rows that do not
    satisfy the condition.  HAVING is different
    from WHERE: WHERE filters
    individual rows before the application of GROUP
    BY, while HAVING filters group rows
    created by GROUP BY.  Each column referenced in
    condition must
    unambiguously reference a grouping column, unless the reference
    appears within an aggregate function or the ungrouped column is
    functionally dependent on the grouping columns.
   
    The presence of HAVING turns a query into a grouped
    query even if there is no GROUP BY clause.  This is the
    same as what happens when the query contains aggregate functions but
    no GROUP BY clause.  All the selected rows are considered to
    form a single group, and the SELECT list and
    HAVING clause can only reference table columns from
    within aggregate functions.  Such a query will emit a single row if the
    HAVING condition is true, zero rows if it is not true.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE and FOR KEY SHARE cannot be
    specified with HAVING.
   
WINDOW Clause
    The optional WINDOW clause has the general form
WINDOWwindow_nameAS (window_definition) [, ...]
    where window_name is
    a name that can be referenced from OVER clauses or
    subsequent window definitions, and
    window_definition is
[existing_window_name] [ PARTITION BYexpression[, ...] ] [ ORDER BYexpression[ ASC | DESC | USINGoperator] [ NULLS { FIRST | LAST } ] [, ...] ] [frame_clause]
    If an existing_window_name
    is specified it must refer to an earlier entry in the WINDOW
    list; the new window copies its partitioning clause from that entry,
    as well as its ordering clause if any.  In this case the new window cannot
    specify its own PARTITION BY clause, and it can specify
    ORDER BY only if the copied window does not have one.
    The new window always uses its own frame clause; the copied window
    must not specify a frame clause.
   
    The elements of the PARTITION BY list are interpreted in
    much the same fashion as elements of a GROUP BY clause, except that
    they are always simple expressions and never the name or number of an
    output column.
    Another difference is that these expressions can contain aggregate
    function calls, which are not allowed in a regular GROUP BY
    clause.  They are allowed here because windowing occurs after grouping
    and aggregation.
   
    Similarly, the elements of the ORDER BY list are interpreted
    in much the same fashion as elements of a statement-level ORDER BY clause, except that
    the expressions are always taken as simple expressions and never the name
    or number of an output column.
   
    The optional frame_clause defines
    the window frame for window functions that depend on the
    frame (not all do).  The window frame is a set of related rows for
    each row of the query (called the current row).
    The frame_clause can be one of
{ RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
{ RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]
    where frame_start
    and frame_end can be one of
UNBOUNDED PRECEDINGoffsetPRECEDING CURRENT ROWoffsetFOLLOWING UNBOUNDED FOLLOWING
    and frame_exclusion can be one of
EXCLUDE CURRENT ROW EXCLUDE GROUP EXCLUDE TIES EXCLUDE NO OTHERS
    If frame_end is omitted it defaults to CURRENT
    ROW.  Restrictions are that
    frame_start cannot be UNBOUNDED FOLLOWING,
    frame_end cannot be UNBOUNDED PRECEDING,
    and the frame_end choice cannot appear earlier in the
    above list of frame_start
    and frame_end options than
    the frame_start choice does — for example
    RANGE BETWEEN CURRENT ROW AND  is not allowed.
   offset
    PRECEDING
    The default framing option is RANGE UNBOUNDED PRECEDING,
    which is the same as RANGE BETWEEN UNBOUNDED PRECEDING AND
    CURRENT ROW; it sets the frame to be all rows from the partition start
    up through the current row's last peer (a row
    that the window's ORDER BY clause considers
    equivalent to the current row; all rows are peers if there
    is no ORDER BY).
    In general, UNBOUNDED PRECEDING means that the frame
    starts with the first row of the partition, and similarly
    UNBOUNDED FOLLOWING means that the frame ends with the last
    row of the partition, regardless
    of RANGE, ROWS
    or GROUPS mode.
    In ROWS mode, CURRENT ROW means
    that the frame starts or ends with the current row; but
    in RANGE or GROUPS mode it means
    that the frame starts or ends with the current row's first or last peer
    in the ORDER BY ordering.
    The offset PRECEDING and
    offset FOLLOWING options
    vary in meaning depending on the frame mode.
    In ROWS mode, the offset
    is an integer indicating that the frame starts or ends that many rows
    before or after the current row.
    In GROUPS mode, the offset
    is an integer indicating that the frame starts or ends that many peer
    groups before or after the current row's peer group, where
    a peer group is a group of rows that are
    equivalent according to the window's ORDER BY clause.
    In RANGE mode, use of
    an offset option requires that there be
    exactly one ORDER BY column in the window definition.
    Then the frame contains those rows whose ordering column value is no
    more than offset less than
    (for PRECEDING) or more than
    (for FOLLOWING) the current row's ordering column
    value.  In these cases the data type of
    the offset expression depends on the data
    type of the ordering column.  For numeric ordering columns it is
    typically of the same type as the ordering column, but for datetime
    ordering columns it is an interval.
    In all these cases, the value of the offset
    must be non-null and non-negative.  Also, while
    the offset does not have to be a simple
    constant, it cannot contain variables, aggregate functions, or window
    functions.
   
    The frame_exclusion option allows rows around
    the current row to be excluded from the frame, even if they would be
    included according to the frame start and frame end options.
    EXCLUDE CURRENT ROW excludes the current row from the
    frame.
    EXCLUDE GROUP excludes the current row and its
    ordering peers from the frame.
    EXCLUDE TIES excludes any peers of the current
    row from the frame, but not the current row itself.
    EXCLUDE NO OTHERS simply specifies explicitly the
    default behavior of not excluding the current row or its peers.
   
    Beware that the ROWS mode can produce unpredictable
    results if the ORDER BY ordering does not order the rows
    uniquely.  The RANGE and GROUPS
    modes are designed to ensure that rows that are peers in
    the ORDER BY ordering are treated alike: all rows of
    a given peer group will be in the frame or excluded from it.
   
    The purpose of a WINDOW clause is to specify the
    behavior of window functions appearing in the query's
    SELECT list or
    ORDER BY clause.
    These functions
    can reference the WINDOW clause entries by name
    in their OVER clauses.  A WINDOW clause
    entry does not have to be referenced anywhere, however; if it is not
    used in the query it is simply ignored.  It is possible to use window
    functions without any WINDOW clause at all, since
    a window function call can specify its window definition directly in
    its OVER clause.  However, the WINDOW
    clause saves typing when the same window definition is needed for more
    than one window function.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE and FOR KEY SHARE cannot be
    specified with WINDOW.
   
Window functions are described in detail in Section 3.5, Section 4.2.8, and Section 7.2.5.
SELECT List
    The SELECT list (between the key words
    SELECT and FROM) specifies expressions
    that form the output rows of the SELECT
    statement.  The expressions can (and usually do) refer to columns
    computed in the FROM clause.
   
    Just as in a table, every output column of a SELECT
    has a name.  In a simple SELECT this name is just
    used to label the column for display, but when the SELECT
    is a sub-query of a larger query, the name is seen by the larger query
    as the column name of the virtual table produced by the sub-query.
    To specify the name to use for an output column, write
    AS output_name
    after the column's expression.  (You can omit AS,
    but only if the desired output name does not match any
    PostgreSQL keyword (see Appendix C).  For protection against possible
    future keyword additions, it is recommended that you always either
    write AS or double-quote the output name.)
    If you do not specify a column name, a name is chosen automatically
    by PostgreSQL.  If the column's expression
    is a simple column reference then the chosen name is the same as that
    column's name.  In more complex cases a function or type name may be
    used, or the system may fall back on a generated name such as
    ?column?.
   
    An output column's name can be used to refer to the column's value in
    ORDER BY and GROUP BY clauses, but not in the
    WHERE or HAVING clauses; there you must write
    out the expression instead.
   
    Instead of an expression, * can be written in
    the output list as a shorthand for all the columns of the selected
    rows.  Also, you can write table_name.*AS;
    the output column names will be the same as the table columns' names.
   
    According to the SQL standard, the expressions in the output list should
    be computed before applying DISTINCT, ORDER
    BY, or LIMIT.  This is obviously necessary
    when using DISTINCT, since otherwise it's not clear
    what values are being made distinct.  However, in many cases it is
    convenient if output expressions are computed after ORDER
    BY and LIMIT; particularly if the output list
    contains any volatile or expensive functions.  With that behavior, the
    order of function evaluations is more intuitive and there will not be
    evaluations corresponding to rows that never appear in the output.
    PostgreSQL will effectively evaluate output expressions
    after sorting and limiting, so long as those expressions are not
    referenced in DISTINCT, ORDER BY
    or GROUP BY.  (As a counterexample, SELECT
    f(x) FROM tab ORDER BY 1 clearly must evaluate f(x)
    before sorting.)  Output expressions that contain set-returning functions
    are effectively evaluated after sorting and before limiting, so
    that LIMIT will act to cut off the output from a
    set-returning function.
   
PostgreSQL versions before 9.6 did not provide any guarantees about the timing of evaluation of output expressions versus sorting and limiting; it depended on the form of the chosen query plan.
DISTINCT Clause
    If SELECT DISTINCT is specified, all duplicate rows are
    removed from the result set (one row is kept from each group of
    duplicates).  SELECT ALL specifies the opposite: all rows are
    kept; that is the default.
   
    SELECT DISTINCT ON ( 
    keeps only the first row of each set of rows where the given
    expressions evaluate to equal.  The expression [, ...] )DISTINCT ON
    expressions are interpreted using the same rules as for
    ORDER BY (see above).  Note that the “first
    row” of each set is unpredictable unless ORDER
    BY is used to ensure that the desired row appears first.  For
    example:
SELECT DISTINCT ON (location) location, time, report
    FROM weather_reports
    ORDER BY location, time DESC;
    retrieves the most recent weather report for each location.  But
    if we had not used ORDER BY to force descending order
    of time values for each location, we'd have gotten a report from
    an unpredictable time for each location.
   
    The DISTINCT ON expression(s) must match the leftmost
    ORDER BY expression(s).  The ORDER BY clause
    will normally contain additional expression(s) that determine the
    desired precedence of rows within each DISTINCT ON group.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE and FOR KEY SHARE cannot be
    specified with DISTINCT.
   
UNION Clause
    The UNION clause has this general form:
select_statementUNION [ ALL | DISTINCT ]select_statement
select_statement is
    any SELECT statement without an ORDER
    BY, LIMIT, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE, or FOR KEY SHARE clause.
    (ORDER BY and LIMIT can be attached to a
    subexpression if it is enclosed in parentheses.  Without
    parentheses, these clauses will be taken to apply to the result of
    the UNION, not to its right-hand input
    expression.)
   
    The UNION operator computes the set union of
    the rows returned by the involved SELECT
    statements.  A row is in the set union of two result sets if it
    appears in at least one of the result sets.  The two
    SELECT statements that represent the direct
    operands of the UNION must produce the same
    number of columns, and corresponding columns must be of compatible
    data types.
   
    The result of UNION does not contain any duplicate
    rows unless the ALL option is specified.
    ALL prevents elimination of duplicates.  (Therefore,
    UNION ALL is usually significantly quicker than
    UNION; use ALL when you can.)
    DISTINCT can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   
    Multiple UNION operators in the same
    SELECT statement are evaluated left to right,
    unless otherwise indicated by parentheses.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and
    FOR KEY SHARE cannot be
    specified either for a UNION result or for any input of a
    UNION.
   
INTERSECT Clause
    The INTERSECT clause has this general form:
select_statementINTERSECT [ ALL | DISTINCT ]select_statement
select_statement is
    any SELECT statement without an ORDER
    BY, LIMIT, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE, or FOR KEY SHARE clause.
   
    The INTERSECT operator computes the set
    intersection of the rows returned by the involved
    SELECT statements.  A row is in the
    intersection of two result sets if it appears in both result sets.
   
    The result of INTERSECT does not contain any
    duplicate rows unless the ALL option is specified.
    With ALL, a row that has m duplicates in the
    left table and n duplicates in the right table will appear
    min(m,n) times in the result set.
    DISTINCT can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   
    Multiple INTERSECT operators in the same
    SELECT statement are evaluated left to right,
    unless parentheses dictate otherwise.
    INTERSECT binds more tightly than
    UNION.  That is, A UNION B INTERSECT
    C will be read as A UNION (B INTERSECT
    C).
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and
    FOR KEY SHARE cannot be
    specified either for an INTERSECT result or for any input of
    an INTERSECT.
   
EXCEPT Clause
    The EXCEPT clause has this general form:
select_statementEXCEPT [ ALL | DISTINCT ]select_statement
select_statement is
    any SELECT statement without an ORDER
    BY, LIMIT, FOR NO KEY UPDATE, FOR UPDATE,
    FOR SHARE, or FOR KEY SHARE clause.
   
    The EXCEPT operator computes the set of rows
    that are in the result of the left SELECT
    statement but not in the result of the right one.
   
    The result of EXCEPT does not contain any
    duplicate rows unless the ALL option is specified.
    With ALL, a row that has m duplicates in the
    left table and n duplicates in the right table will appear
    max(m-n,0) times in the result set.
    DISTINCT can be written to explicitly specify the
    default behavior of eliminating duplicate rows.
   
    Multiple EXCEPT operators in the same
    SELECT statement are evaluated left to right,
    unless parentheses dictate otherwise.  EXCEPT binds at
    the same level as UNION.
   
    Currently, FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE and
    FOR KEY SHARE cannot be
    specified either for an EXCEPT result or for any input of
    an EXCEPT.
   
ORDER BY Clause
    The optional ORDER BY clause has this general form:
ORDER BYexpression[ ASC | DESC | USINGoperator] [ NULLS { FIRST | LAST } ] [, ...]
    The ORDER BY clause causes the result rows to
    be sorted according to the specified expression(s).  If two rows are
    equal according to the leftmost expression, they are compared
    according to the next expression and so on.  If they are equal
    according to all specified expressions, they are returned in
    an implementation-dependent order.
   
    Each expression can be the
    name or ordinal number of an output column
    (SELECT list item), or it can be an arbitrary
    expression formed from input-column values.
   
    The ordinal number refers to the ordinal (left-to-right) position
    of the output column. This feature makes it possible to define an
    ordering on the basis of a column that does not have a unique
    name.  This is never absolutely necessary because it is always
    possible to assign a name to an output column using the
    AS clause.
   
    It is also possible to use arbitrary expressions in the
    ORDER BY clause, including columns that do not
    appear in the SELECT output list.  Thus the
    following statement is valid:
SELECT name FROM distributors ORDER BY code;
    A limitation of this feature is that an ORDER BY
    clause applying to the result of a UNION,
    INTERSECT, or EXCEPT clause can only
    specify an output column name or number, not an expression.
   
    If an ORDER BY expression is a simple name that
    matches both an output column name and an input column name,
    ORDER BY will interpret it as the output column name.
    This is the opposite of the choice that GROUP BY will
    make in the same situation.  This inconsistency is made to be
    compatible with the SQL standard.
   
    Optionally one can add the key word ASC (ascending) or
    DESC (descending) after any expression in the
    ORDER BY clause.  If not specified, ASC is
    assumed by default.  Alternatively, a specific ordering operator
    name can be specified in the USING clause.
    An ordering operator must be a less-than or greater-than
    member of some B-tree operator family.
    ASC is usually equivalent to USING < and
    DESC is usually equivalent to USING >.
    (But the creator of a user-defined data type can define exactly what the
    default sort ordering is, and it might correspond to operators with other
    names.)
   
    If NULLS LAST is specified, null values sort after all
    non-null values; if NULLS FIRST is specified, null values
    sort before all non-null values.  If neither is specified, the default
    behavior is NULLS LAST when ASC is specified
    or implied, and NULLS FIRST when DESC is specified
    (thus, the default is to act as though nulls are larger than non-nulls).
    When USING is specified, the default nulls ordering depends
    on whether the operator is a less-than or greater-than operator.
   
    Note that ordering options apply only to the expression they follow;
    for example ORDER BY x, y DESC does not mean
    the same thing as ORDER BY x DESC, y DESC.
   
    Character-string data is sorted according to the collation that applies
    to the column being sorted.  That can be overridden at need by including
    a COLLATE clause in the
    expression, for example
    ORDER BY mycolumn COLLATE "en_US".
    For more information see Section 4.2.10 and
    Section 24.2.
   
LIMIT Clause
    The LIMIT clause consists of two independent
    sub-clauses:
LIMIT { count | ALL }
OFFSET start
    The parameter count specifies the
    maximum number of rows to return, while start specifies the number of rows
    to skip before starting to return rows.  When both are specified,
    start rows are skipped
    before starting to count the count rows to be returned.
   
    If the count expression
    evaluates to NULL, it is treated as LIMIT ALL, i.e., no
    limit.  If start evaluates
    to NULL, it is treated the same as OFFSET 0.
   
SQL:2008 introduced a different syntax to achieve the same result, which PostgreSQL also supports. It is:
OFFSETstart{ ROW | ROWS } FETCH { FIRST | NEXT } [count] { ROW | ROWS } { ONLY | WITH TIES }
    In this syntax, the start
    or count value is required by
    the standard to be a literal constant, a parameter, or a variable name;
    as a PostgreSQL extension, other expressions
    are allowed, but will generally need to be enclosed in parentheses to avoid
    ambiguity.
    If count is
    omitted in a FETCH clause, it defaults to 1.
    The WITH TIES option is used to return any additional
    rows that tie for the last place in the result set according to
    the ORDER BY clause; ORDER BY
    is mandatory in this case, and SKIP LOCKED is
    not allowed.
    ROW and ROWS as well as
    FIRST and NEXT are noise
    words that don't influence the effects of these clauses.
    According to the standard, the OFFSET clause must come
    before the FETCH clause if both are present; but
    PostgreSQL is laxer and allows either order.
   
    When using LIMIT, it is a good idea to use an
    ORDER BY clause that constrains the result rows into a
    unique order.  Otherwise you will get an unpredictable subset of
    the query's rows — you might be asking for the tenth through
    twentieth rows, but tenth through twentieth in what ordering?  You
    don't know what ordering unless you specify ORDER BY.
   
    The query planner takes LIMIT into account when
    generating a query plan, so you are very likely to get different
    plans (yielding different row orders) depending on what you use
    for LIMIT and OFFSET.  Thus, using
    different LIMIT/OFFSET values to select
    different subsets of a query result will give
    inconsistent results unless you enforce a predictable
    result ordering with ORDER BY.  This is not a bug; it
    is an inherent consequence of the fact that SQL does not promise
    to deliver the results of a query in any particular order unless
    ORDER BY is used to constrain the order.
   
    It is even possible for repeated executions of the same LIMIT
    query to return different subsets of the rows of a table, if there
    is not an ORDER BY to enforce selection of a deterministic
    subset.  Again, this is not a bug; determinism of the results is
    simply not guaranteed in such a case.
   
    FOR UPDATE, FOR NO KEY UPDATE, FOR SHARE
    and FOR KEY SHARE
    are locking clauses; they affect how SELECT
    locks rows as they are obtained from the table.
   
The locking clause has the general form
FORlock_strength[ OFtable_name[, ...] ] [ NOWAIT | SKIP LOCKED ]
    where lock_strength can be one of
UPDATE NO KEY UPDATE SHARE KEY SHARE
For more information on each row-level lock mode, refer to Section 13.3.2.
    To prevent the operation from waiting for other transactions to commit,
    use either the NOWAIT or SKIP LOCKED
    option.  With NOWAIT, the statement reports an error, rather
    than waiting, if a selected row cannot be locked immediately.
    With SKIP LOCKED, any selected rows that cannot be
    immediately locked are skipped.  Skipping locked rows provides an
    inconsistent view of the data, so this is not suitable for general purpose
    work, but can be used to avoid lock contention with multiple consumers
    accessing a queue-like table.
    Note that NOWAIT and SKIP LOCKED apply only
    to the row-level lock(s) — the required ROW SHARE
    table-level lock is still taken in the ordinary way (see
    Chapter 13).  You can use
    LOCK
    with the NOWAIT option first,
    if you need to acquire the table-level lock without waiting.
   
    If specific tables are named in a locking clause,
    then only rows coming from those tables are locked; any other
    tables used in the SELECT are simply read as
    usual.  A locking
    clause without a table list affects all tables used in the statement.
    If a locking clause is
    applied to a view or sub-query, it affects all tables used in
    the view or sub-query.
    However, these clauses
    do not apply to WITH queries referenced by the primary query.
    If you want row locking to occur within a WITH query, specify
    a locking clause within the WITH query.
   
    Multiple locking
    clauses can be written if it is necessary to specify different locking
    behavior for different tables.  If the same table is mentioned (or
    implicitly affected) by more than one locking clause,
    then it is processed as if it was only specified by the strongest one.
    Similarly, a table is processed
    as NOWAIT if that is specified in any of the clauses
    affecting it.  Otherwise, it is processed
    as SKIP LOCKED if that is specified in any of the
    clauses affecting it.
   
The locking clauses cannot be used in contexts where returned rows cannot be clearly identified with individual table rows; for example they cannot be used with aggregation.
    When a locking clause
    appears at the top level of a SELECT query, the rows that
    are locked are exactly those that are returned by the query; in the
    case of a join query, the rows locked are those that contribute to
    returned join rows.  In addition, rows that satisfied the query
    conditions as of the query snapshot will be locked, although they
    will not be returned if they were updated after the snapshot
    and no longer satisfy the query conditions.  If a
    LIMIT is used, locking stops
    once enough rows have been returned to satisfy the limit (but note that
    rows skipped over by OFFSET will get locked).  Similarly,
    if a locking clause
    is used in a cursor's query, only rows actually fetched or stepped past
    by the cursor will be locked.
   
    When a locking clause
    appears in a sub-SELECT, the rows locked are those
    returned to the outer query by the sub-query.  This might involve
    fewer rows than inspection of the sub-query alone would suggest,
    since conditions from the outer query might be used to optimize
    execution of the sub-query.  For example,
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;
    will lock only rows having col1 = 5, even though that
    condition is not textually within the sub-query.
   
Previous releases failed to preserve a lock which is upgraded by a later savepoint. For example, this code:
BEGIN; SELECT * FROM mytable WHERE key = 1 FOR UPDATE; SAVEPOINT s; UPDATE mytable SET ... WHERE key = 1; ROLLBACK TO s;
   would fail to preserve the FOR UPDATE lock after the
   ROLLBACK TO.  This has been fixed in release 9.3.
  
    It is possible for a SELECT command running at the READ
    COMMITTED transaction isolation level and using ORDER
    BY and a locking clause to return rows out of
    order.  This is because ORDER BY is applied first.
    The command sorts the result, but might then block trying to obtain a lock
    on one or more of the rows.  Once the SELECT unblocks, some
    of the ordering column values might have been modified, leading to those
    rows appearing to be out of order (though they are in order in terms
    of the original column values).  This can be worked around at need by
    placing the FOR UPDATE/SHARE clause in a sub-query,
    for example
SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;
    Note that this will result in locking all rows of mytable,
    whereas FOR UPDATE at the top level would lock only the
    actually returned rows.  This can make for a significant performance
    difference, particularly if the ORDER BY is combined with
    LIMIT or other restrictions.  So this technique is recommended
    only if concurrent updates of the ordering columns are expected and a
    strictly sorted result is required.
   
    At the REPEATABLE READ or SERIALIZABLE
    transaction isolation level this would cause a serialization failure (with
    an SQLSTATE of '40001'), so there is
    no possibility of receiving rows out of order under these isolation levels.
   
TABLE CommandThe command
TABLE name
is equivalent to
SELECT * FROM name
    It can be used as a top-level command or as a space-saving syntax
    variant in parts of complex queries. Only the WITH,
    UNION, INTERSECT, EXCEPT,
    ORDER BY, LIMIT, OFFSET,
    FETCH and FOR locking clauses can be used
    with TABLE; the WHERE clause and any form of
    aggregation cannot
    be used.
   
   To join the table films with the table
   distributors:
SELECT f.title, f.did, d.name, f.date_prod, f.kind
    FROM distributors d JOIN films f USING (did);
       title       | did |     name     | date_prod  |   kind
-------------------+-----+--------------+------------+----------
 The Third Man     | 101 | British Lion | 1949-12-23 | Drama
 The African Queen | 101 | British Lion | 1951-08-11 | Romantic
 ...
   To sum the column len of all films and group
   the results by kind:
SELECT kind, sum(len) AS total FROM films GROUP BY kind; kind | total ----------+------- Action | 07:34 Comedy | 02:58 Drama | 14:28 Musical | 06:42 Romantic | 04:38
   To sum the column len of all films, group
   the results by kind and show those group totals
   that are less than 5 hours:
SELECT kind, sum(len) AS total
    FROM films
    GROUP BY kind
    HAVING sum(len) < interval '5 hours';
   kind   | total
----------+-------
 Comedy   | 02:58
 Romantic | 04:38
   The following two examples are identical ways of sorting the individual
   results according to the contents of the second column
   (name):
SELECT * FROM distributors ORDER BY name; SELECT * FROM distributors ORDER BY 2; did | name -----+------------------ 109 | 20th Century Fox 110 | Bavaria Atelier 101 | British Lion 107 | Columbia 102 | Jean Luc Godard 113 | Luso films 104 | Mosfilm 103 | Paramount 106 | Toho 105 | United Artists 111 | Walt Disney 112 | Warner Bros. 108 | Westward
   The next example shows how to obtain the union of the tables
   distributors and
   actors, restricting the results to those that begin
   with the letter W in each table.  Only distinct rows are wanted, so the
   key word ALL is omitted.
distributors:               actors:
 did |     name              id |     name
-----+--------------        ----+----------------
 108 | Westward               1 | Woody Allen
 111 | Walt Disney            2 | Warren Beatty
 112 | Warner Bros.           3 | Walter Matthau
 ...                         ...
SELECT distributors.name
    FROM distributors
    WHERE distributors.name LIKE 'W%'
UNION
SELECT actors.name
    FROM actors
    WHERE actors.name LIKE 'W%';
      name
----------------
 Walt Disney
 Walter Matthau
 Warner Bros.
 Warren Beatty
 Westward
 Woody Allen
   This example shows how to use a function in the FROM
   clause, both with and without a column definition list:
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
    SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors(111);
 did |    name
-----+-------------
 111 | Walt Disney
CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
    SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
 f1  |     f2
-----+-------------
 111 | Walt Disney
Here is an example of a function with an ordinality column added:
SELECT * FROM unnest(ARRAY['a','b','c','d','e','f']) WITH ORDINALITY; unnest | ordinality --------+---------- a | 1 b | 2 c | 3 d | 4 e | 5 f | 6 (6 rows)
   This example shows how to use a simple WITH clause:
WITH t AS (
    SELECT random() as x FROM generate_series(1, 3)
  )
SELECT * FROM t
UNION ALL
SELECT * FROM t;
         x
--------------------
  0.534150459803641
  0.520092216785997
 0.0735620250925422
  0.534150459803641
  0.520092216785997
 0.0735620250925422
   Notice that the WITH query was evaluated only once,
   so that we got two sets of the same three random values.
  
   This example uses WITH RECURSIVE to find all
   subordinates (direct or indirect) of the employee Mary, and their
   level of indirectness, from a table that shows only direct
   subordinates:
WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
    SELECT 1, employee_name, manager_name
    FROM employee
    WHERE manager_name = 'Mary'
  UNION ALL
    SELECT er.distance + 1, e.employee_name, e.manager_name
    FROM employee_recursive er, employee e
    WHERE er.employee_name = e.manager_name
  )
SELECT distance, employee_name FROM employee_recursive;
   Notice the typical form of recursive queries:
   an initial condition, followed by UNION,
   followed by the recursive part of the query. Be sure that the
   recursive part of the query will eventually return no tuples, or
   else the query will loop indefinitely.  (See Section 7.8
   for more examples.)
  
   This example uses LATERAL to apply a set-returning function
   get_product_names() for each row of the
   manufacturers table:
SELECT m.name AS mname, pname FROM manufacturers m, LATERAL get_product_names(m.id) pname;
Manufacturers not currently having any products would not appear in the result, since it is an inner join. If we wished to include the names of such manufacturers in the result, we could do:
SELECT m.name AS mname, pname FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true;
   Of course, the SELECT statement is compatible
   with the SQL standard.  But there are some extensions and some
   missing features.
  
FROM Clauses
    PostgreSQL allows one to omit the
    FROM clause.  It has a straightforward use to
    compute the results of simple expressions:
SELECT 2+2;
 ?column?
----------
        4
    Some other SQL databases cannot do this except
    by introducing a dummy one-row table from which to do the
    SELECT.
   
SELECT Lists
    The list of output expressions after SELECT can be
    empty, producing a zero-column result table.
    This is not valid syntax according to the SQL standard.
    PostgreSQL allows it to be consistent with
    allowing zero-column tables.
    However, an empty list is not allowed when DISTINCT is used.
   
AS Key Word
    In the SQL standard, the optional key word AS can be
    omitted before an output column name whenever the new column name
    is a valid column name (that is, not the same as any reserved
    keyword).  PostgreSQL is slightly more
    restrictive: AS is required if the new column name
    matches any keyword at all, reserved or not.  Recommended practice is
    to use AS or double-quote output column names, to prevent
    any possible conflict against future keyword additions.
   
    In FROM items, both the standard and
    PostgreSQL allow AS to
    be omitted before an alias that is an unreserved keyword.  But
    this is impractical for output column names, because of syntactic
    ambiguities.
   
ONLY and Inheritance
    The SQL standard requires parentheses around the table name when
    writing ONLY, for example SELECT * FROM ONLY
    (tab1), ONLY (tab2) WHERE ....  PostgreSQL
    considers these parentheses to be optional.
   
    PostgreSQL allows a trailing * to be written to
    explicitly specify the non-ONLY behavior of including
    child tables.  The standard does not allow this.
   
    (These points apply equally to all SQL commands supporting the
    ONLY option.)
   
TABLESAMPLE Clause Restrictions
    The TABLESAMPLE clause is currently accepted only on
    regular tables and materialized views.  According to the SQL standard
    it should be possible to apply it to any FROM item.
   
FROM
    PostgreSQL allows a function call to be
    written directly as a member of the FROM list.  In the SQL
    standard it would be necessary to wrap such a function call in a
    sub-SELECT; that is, the syntax
    FROM 
    is approximately equivalent to
    func(...) aliasFROM LATERAL (SELECT .
    Note that func(...)) aliasLATERAL is considered to be implicit; this is
    because the standard requires LATERAL semantics for an
    UNNEST() item in FROM.
    PostgreSQL treats UNNEST() the
    same as other set-returning functions.
   
GROUP BY and ORDER BY
    In the SQL-92 standard, an ORDER BY clause can
    only use output column names or numbers, while a GROUP
    BY clause can only use expressions based on input column
    names.  PostgreSQL extends each of
    these clauses to allow the other choice as well (but it uses the
    standard's interpretation if there is ambiguity).
    PostgreSQL also allows both clauses to
    specify arbitrary expressions.  Note that names appearing in an
    expression will always be taken as input-column names, not as
    output-column names.
   
    SQL:1999 and later use a slightly different definition which is not
    entirely upward compatible with SQL-92.
    In most cases, however, PostgreSQL
    will interpret an ORDER BY or GROUP
    BY expression the same way SQL:1999 does.
   
    PostgreSQL recognizes functional dependency
    (allowing columns to be omitted from GROUP BY) only when
    a table's primary key is included in the GROUP BY list.
    The SQL standard specifies additional conditions that should be
    recognized.
   
LIMIT and OFFSET
    The clauses LIMIT and OFFSET
    are PostgreSQL-specific syntax, also
    used by MySQL.  The SQL:2008 standard
    has introduced the clauses OFFSET ... FETCH {FIRST|NEXT}
    ... for the same functionality, as shown above
    in LIMIT Clause.  This
    syntax is also used by IBM DB2.
    (Applications written for Oracle
    frequently use a workaround involving the automatically
    generated rownum column, which is not available in
    PostgreSQL, to implement the effects of these clauses.)
   
FOR NO KEY UPDATE, FOR UPDATE, FOR SHARE, FOR KEY SHARE
    Although FOR UPDATE appears in the SQL standard, the
    standard allows it only as an option of DECLARE CURSOR.
    PostgreSQL allows it in any SELECT
    query as well as in sub-SELECTs, but this is an extension.
    The FOR NO KEY UPDATE, FOR SHARE and
    FOR KEY SHARE variants, as well as the NOWAIT
    and SKIP LOCKED options, do not appear in the
    standard.
   
WITH
    PostgreSQL allows INSERT,
    UPDATE, and DELETE to be used as WITH
    queries.  This is not found in the SQL standard.
   
    DISTINCT ON ( ... ) is an extension of the
    SQL standard.
   
    ROWS FROM( ... ) is an extension of the SQL standard.
   
    The MATERIALIZED and NOT
    MATERIALIZED options of WITH are extensions
    of the SQL standard.