SQL input consists of a sequence of commands. A command is composed of a sequence of tokens, terminated by a semicolon (“;”). The end of the input stream also terminates a command. Which tokens are valid depends on the syntax of the particular command.
A token can be a key word, an identifier, a quoted identifier, a literal (or constant), or a special character symbol. Tokens are normally separated by whitespace (space, tab, newline), but need not be if there is no ambiguity (which is generally only the case if a special character is adjacent to some other token type).
For example, the following is (syntactically) valid SQL input:
SELECT * FROM MY_TABLE; UPDATE MY_TABLE SET A = 5; INSERT INTO MY_TABLE VALUES (3, 'hi there');
This is a sequence of three commands, one per line (although this is not required; more than one command can be on a line, and commands can usefully be split across lines).
Additionally, comments can occur in SQL input. They are not tokens, they are effectively equivalent to whitespace.
   The SQL syntax is not very consistent regarding what tokens
   identify commands and which are operands or parameters.  The first
   few tokens are generally the command name, so in the above example
   we would usually speak of a “SELECT”, an
   “UPDATE”, and an “INSERT” command.  But
   for instance the UPDATE command always requires
   a SET token to appear in a certain position, and
   this particular variation of INSERT also
   requires a VALUES in order to be complete.  The
   precise syntax rules for each command are described in Part VI.
  
    Tokens such as SELECT, UPDATE, or
    VALUES in the example above are examples of
    key words, that is, words that have a fixed
    meaning in the SQL language.  The tokens MY_TABLE
    and A are examples of
    identifiers.  They identify names of
    tables, columns, or other database objects, depending on the
    command they are used in.  Therefore they are sometimes simply
    called “names”.  Key words and identifiers have the
    same lexical structure, meaning that one cannot know whether a
    token is an identifier or a key word without knowing the language.
    A complete list of key words can be found in Appendix C.
   
    SQL identifiers and key words must begin with a letter
    (a-z, but also letters with
    diacritical marks and non-Latin letters) or an underscore
    (_).  Subsequent characters in an identifier or
    key word can be letters, underscores, digits
    (0-9), or dollar signs
    ($).  Note that dollar signs are not allowed in identifiers
    according to the letter of the SQL standard, so their use might render
    applications less portable.
    The SQL standard will not define a key word that contains
    digits or starts or ends with an underscore, so identifiers of this
    form are safe against possible conflict with future extensions of the
    standard.
   
    
    The system uses no more than NAMEDATALEN-1
    bytes of an identifier; longer names can be written in
    commands, but they will be truncated.  By default,
    NAMEDATALEN is 64 so the maximum identifier
    length is 63 bytes. If this limit is problematic, it can be raised by
    changing the NAMEDATALEN constant in
    src/include/pg_config_manual.h.
   
Key words and unquoted identifiers are case insensitive. Therefore:
UPDATE MY_TABLE SET A = 5;
can equivalently be written as:
uPDaTE my_TabLE SeT a = 5;
A convention often used is to write key words in upper case and names in lower case, e.g.:
UPDATE my_table SET a = 5;
    
    There is a second kind of identifier:  the delimited
    identifier or quoted
    identifier.  It is formed by enclosing an arbitrary
    sequence of characters in double-quotes
    (").  A delimited
    identifier is always an identifier, never a key word.  So
    "select" could be used to refer to a column or
    table named “select”, whereas an unquoted
    select would be taken as a key word and
    would therefore provoke a parse error when used where a table or
    column name is expected.  The example can be written with quoted
    identifiers like this:
UPDATE "my_table" SET "a" = 5;
Quoted identifiers can contain any character, except the character with code zero. (To include a double quote, write two double quotes.) This allows constructing table or column names that would otherwise not be possible, such as ones containing spaces or ampersands. The length limitation still applies.
    A variant of quoted
    identifiers allows including escaped Unicode characters identified
    by their code points.  This variant starts
    with U& (upper or lower case U followed by
    ampersand) immediately before the opening double quote, without
    any spaces in between, for example U&"foo".
    (Note that this creates an ambiguity with the
    operator &.  Use spaces around the operator to
    avoid this problem.)  Inside the quotes, Unicode characters can be
    specified in escaped form by writing a backslash followed by the
    four-digit hexadecimal code point number or alternatively a
    backslash followed by a plus sign followed by a six-digit
    hexadecimal code point number.  For example, the
    identifier "data" could be written as
U&"d\0061t\+000061"
The following less trivial example writes the Russian word “slon” (elephant) in Cyrillic letters:
U&"\0441\043B\043E\043D"
    If a different escape character than backslash is desired, it can
    be specified using
    the UESCAPE
    clause after the string, for example:
U&"d!0061t!+000061" UESCAPE '!'
The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character. Note that the escape character is written in single quotes, not double quotes.
To include the escape character in the identifier literally, write it twice.
    The Unicode escape syntax works only when the server encoding is
    UTF8.  When other server encodings are used, only code
    points in the ASCII range (up to \007F) can be
    specified.  Both the 4-digit and the 6-digit form can be used to
    specify UTF-16 surrogate pairs to compose characters with code
    points larger than U+FFFF, although the availability of the
    6-digit form technically makes this unnecessary.  (Surrogate
    pairs are not stored directly, but combined into a single
    code point that is then encoded in UTF-8.)
   
    Quoting an identifier also makes it case-sensitive, whereas
    unquoted names are always folded to lower case.  For example, the
    identifiers FOO, foo, and
    "foo" are considered the same by
    PostgreSQL, but
    "Foo" and "FOO" are
    different from these three and each other.  (The folding of
    unquoted names to lower case in PostgreSQL is
    incompatible with the SQL standard, which says that unquoted names
    should be folded to upper case.  Thus, foo
    should be equivalent to "FOO" not
    "foo" according to the standard.  If you want
    to write portable applications you are advised to always quote a
    particular name or never quote it.)
   
There are three kinds of implicitly-typed constants in PostgreSQL: strings, bit strings, and numbers. Constants can also be specified with explicit types, which can enable more accurate representation and more efficient handling by the system. These alternatives are discussed in the following subsections.
     
     A string constant in SQL is an arbitrary sequence of characters
     bounded by single quotes ('), for example
     'This is a string'.  To include
     a single-quote character within a string constant,
     write two adjacent single quotes, e.g.,
     'Dianne''s horse'.
     Note that this is not the same as a double-quote
     character ("). 
    
Two string constants that are only separated by whitespace with at least one newline are concatenated and effectively treated as if the string had been written as one constant. For example:
SELECT 'foo' 'bar';
is equivalent to:
SELECT 'foobar';
but:
SELECT 'foo' 'bar';
is not valid syntax. (This slightly bizarre behavior is specified by SQL; PostgreSQL is following the standard.)
     PostgreSQL also accepts “escape”
     string constants, which are an extension to the SQL standard.
     An escape string constant is specified by writing the letter
     E (upper or lower case) just before the opening single
     quote, e.g., E'foo'.  (When continuing an escape string
     constant across lines, write E only before the first opening
     quote.)
     Within an escape string, a backslash character (\) begins a
     C-like backslash escape sequence, in which the combination
     of backslash and following character(s) represent a special byte
     value, as shown in Table 4.1.
    
Table 4.1. Backslash Escape Sequences
| Backslash Escape Sequence | Interpretation | 
|---|---|
| \b | backspace | 
| \f | form feed | 
| \n | newline | 
| \r | carriage return | 
| \t | tab | 
| \,\,\(o= 0 - 7) | octal byte value | 
| \x,\x(h= 0 - 9, A - F) | hexadecimal byte value | 
| \u,\U(x= 0 - 9, A - F) | 16 or 32-bit hexadecimal Unicode character value | 
     Any other
     character following a backslash is taken literally. Thus, to
     include a backslash character, write two backslashes (\\).
     Also, a single quote can be included in an escape string by writing
     \', in addition to the normal way of ''.
    
It is your responsibility that the byte sequences you create, especially when using the octal or hexadecimal escapes, compose valid characters in the server character set encoding. When the server encoding is UTF-8, then the Unicode escapes or the alternative Unicode escape syntax, explained in Section 4.1.2.3, should be used instead. (The alternative would be doing the UTF-8 encoding by hand and writing out the bytes, which would be very cumbersome.)
     The Unicode escape syntax works fully only when the server
     encoding is UTF8.  When other server encodings are
     used, only code points in the ASCII range (up
     to \u007F) can be specified.  Both the 4-digit and
     the 8-digit form can be used to specify UTF-16 surrogate pairs to
     compose characters with code points larger than U+FFFF, although
     the availability of the 8-digit form technically makes this
     unnecessary.  (When surrogate pairs are used when the server
     encoding is UTF8, they are first combined into a
     single code point that is then encoded in UTF-8.)
    
     If the configuration parameter
     standard_conforming_strings is off,
     then PostgreSQL recognizes backslash escapes
     in both regular and escape string constants.  However, as of
     PostgreSQL 9.1, the default is on, meaning
     that backslash escapes are recognized only in escape string constants.
     This behavior is more standards-compliant, but might break applications
     which rely on the historical behavior, where backslash escapes
     were always recognized.  As a workaround, you can set this parameter
     to off, but it is better to migrate away from using backslash
     escapes.  If you need to use a backslash escape to represent a special
     character, write the string constant with an E.
    
     In addition to standard_conforming_strings, the configuration
     parameters escape_string_warning and
     backslash_quote govern treatment of backslashes
     in string constants.
    
The character with the code zero cannot be in a string constant.
     PostgreSQL also supports another type
     of escape syntax for strings that allows specifying arbitrary
     Unicode characters by code point.  A Unicode escape string
     constant starts with U& (upper or lower case
     letter U followed by ampersand) immediately before the opening
     quote, without any spaces in between, for
     example U&'foo'.  (Note that this creates an
     ambiguity with the operator &.  Use spaces
     around the operator to avoid this problem.)  Inside the quotes,
     Unicode characters can be specified in escaped form by writing a
     backslash followed by the four-digit hexadecimal code point
     number or alternatively a backslash followed by a plus sign
     followed by a six-digit hexadecimal code point number.  For
     example, the string 'data' could be written as
U&'d\0061t\+000061'
The following less trivial example writes the Russian word “slon” (elephant) in Cyrillic letters:
U&'\0441\043B\043E\043D'
     If a different escape character than backslash is desired, it can
     be specified using
     the UESCAPE
     clause after the string, for example:
U&'d!0061t!+000061' UESCAPE '!'
The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character.
     The Unicode escape syntax works only when the server encoding is
     UTF8.  When other server encodings are used, only
     code points in the ASCII range (up to \007F)
     can be specified.  Both the 4-digit and the 6-digit form can be
     used to specify UTF-16 surrogate pairs to compose characters with
     code points larger than U+FFFF, although the availability of the
     6-digit form technically makes this unnecessary.  (When surrogate
     pairs are used when the server encoding is UTF8, they
     are first combined into a single code point that is then encoded
     in UTF-8.)
    
Also, the Unicode escape syntax for string constants only works when the configuration parameter standard_conforming_strings is turned on. This is because otherwise this syntax could confuse clients that parse the SQL statements to the point that it could lead to SQL injections and similar security issues. If the parameter is set to off, this syntax will be rejected with an error message.
To include the escape character in the string literally, write it twice.
     While the standard syntax for specifying string constants is usually
     convenient, it can be difficult to understand when the desired string
     contains many single quotes, since each of those must
     be doubled. To allow more readable queries in such situations,
     PostgreSQL provides another way, called
     “dollar quoting”, to write string constants.
     A dollar-quoted string constant
     consists of a dollar sign ($), an optional
     “tag” of zero or more characters, another dollar
     sign, an arbitrary sequence of characters that makes up the
     string content, a dollar sign, the same tag that began this
     dollar quote, and a dollar sign. For example, here are two
     different ways to specify the string “Dianne's horse”
     using dollar quoting:
$$Dianne's horse$$ $SomeTag$Dianne's horse$SomeTag$
Notice that inside the dollar-quoted string, single quotes can be used without needing to be escaped. Indeed, no characters inside a dollar-quoted string are ever escaped: the string content is always written literally. Backslashes are not special, and neither are dollar signs, unless they are part of a sequence matching the opening tag.
It is possible to nest dollar-quoted string constants by choosing different tags at each nesting level. This is most commonly used in writing function definitions. For example:
$function$
BEGIN
    RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
END;
$function$
     Here, the sequence $q$[\t\r\n\v\\]$q$ represents a
     dollar-quoted literal string [\t\r\n\v\\], which will
     be recognized when the function body is executed by
     PostgreSQL.  But since the sequence does not match
     the outer dollar quoting delimiter $function$, it is
     just some more characters within the constant so far as the outer
     string is concerned.
    
     The tag, if any, of a dollar-quoted string follows the same rules
     as an unquoted identifier, except that it cannot contain a dollar sign.
     Tags are case sensitive, so $tag$String content$tag$
     is correct, but $TAG$String content$tag$ is not.
    
A dollar-quoted string that follows a keyword or identifier must be separated from it by whitespace; otherwise the dollar quoting delimiter would be taken as part of the preceding identifier.
Dollar quoting is not part of the SQL standard, but it is often a more convenient way to write complicated string literals than the standard-compliant single quote syntax. It is particularly useful when representing string constants inside other constants, as is often needed in procedural function definitions. With single-quote syntax, each backslash in the above example would have to be written as four backslashes, which would be reduced to two backslashes in parsing the original string constant, and then to one when the inner string constant is re-parsed during function execution.
     Bit-string constants look like regular string constants with a
     B (upper or lower case) immediately before the
     opening quote (no intervening whitespace), e.g.,
     B'1001'.  The only characters allowed within
     bit-string constants are 0 and
     1.
    
     Alternatively, bit-string constants can be specified in hexadecimal
     notation, using a leading X (upper or lower case),
     e.g., X'1FF'.  This notation is equivalent to
     a bit-string constant with four binary digits for each hexadecimal digit.
    
Both forms of bit-string constant can be continued across lines in the same way as regular string constants. Dollar quoting cannot be used in a bit-string constant.
Numeric constants are accepted in these general forms:
digitsdigits.[digits][e[+-]digits] [digits].digits[e[+-]digits]digitse[+-]digits
     where digits is one or more decimal
     digits (0 through 9).  At least one digit must be before or after the
     decimal point, if one is used.  At least one digit must follow the
     exponent marker (e), if one is present.
     There cannot be any spaces or other characters embedded in the
     constant.  Note that any leading plus or minus sign is not actually
     considered part of the constant; it is an operator applied to the
     constant.
    
These are some examples of valid numeric constants:
42
3.5
4.
.001
5e2
1.925e-3
     
     
     
     A numeric constant that contains neither a decimal point nor an
     exponent is initially presumed to be type integer if its
     value fits in type integer (32 bits); otherwise it is
     presumed to be type bigint if its
     value fits in type bigint (64 bits); otherwise it is
     taken to be type numeric.  Constants that contain decimal
     points and/or exponents are always initially presumed to be type
     numeric.
    
     The initially assigned data type of a numeric constant is just a
     starting point for the type resolution algorithms.  In most cases
     the constant will be automatically coerced to the most
     appropriate type depending on context.  When necessary, you can
     force a numeric value to be interpreted as a specific data type
     by casting it.
     For example, you can force a numeric value to be treated as type
     real (float4) by writing:
REAL '1.23' -- string style 1.23::REAL -- PostgreSQL (historical) style
These are actually just special cases of the general casting notations discussed next.
A constant of an arbitrary type can be entered using any one of the following notations:
type'string' 'string'::typeCAST ( 'string' AStype)
     The string constant's text is passed to the input conversion
     routine for the type called type. The
     result is a constant of the indicated type.  The explicit type
     cast can be omitted if there is no ambiguity as to the type the
     constant must be (for example, when it is assigned directly to a
     table column), in which case it is automatically coerced.
    
The string constant can be written using either regular SQL notation or dollar-quoting.
It is also possible to specify a type coercion using a function-like syntax:
typename( 'string' )
but not all type names can be used in this way; see Section 4.2.9 for details.
     The ::, CAST(), and
     function-call syntaxes can also be used to specify run-time type
     conversions of arbitrary expressions, as discussed in Section 4.2.9.  To avoid syntactic ambiguity, the
     type 'string'type 'string'::
     or CAST() to specify the type of an array constant.
    
     The CAST() syntax conforms to SQL.  The
     type 'string':: is historical PostgreSQL
     usage, as is the function-call syntax.
    
    An operator name is a sequence of up to NAMEDATALEN-1
    (63 by default) characters from the following list:
+ - * / < > = ~ ! @ # % ^ & | ` ?
There are a few restrictions on operator names, however:
       -- and /* cannot appear
       anywhere in an operator name, since they will be taken as the
       start of a comment.
      
       A multiple-character operator name cannot end in + or -,
       unless the name also contains at least one of these characters:
~ ! @ # % ^ & | ` ?
       For example, @- is an allowed operator name,
       but *- is not.  This restriction allows
       PostgreSQL to parse SQL-compliant
       queries without requiring spaces between tokens.
      
    When working with non-SQL-standard operator names, you will usually
    need to separate adjacent operators with spaces to avoid ambiguity.
    For example, if you have defined a left unary operator named @,
    you cannot write X*@Y; you must write
    X* @Y to ensure that
    PostgreSQL reads it as two operator names
    not one.
   
Some characters that are not alphanumeric have a special meaning that is different from being an operator. Details on the usage can be found at the location where the respective syntax element is described. This section only exists to advise the existence and summarize the purposes of these characters.
      A dollar sign ($) followed by digits is used
      to represent a positional parameter in the body of a function
      definition or a prepared statement.  In other contexts the
      dollar sign can be part of an identifier or a dollar-quoted string
      constant.
     
      Parentheses (()) have their usual meaning to
      group expressions and enforce precedence.  In some cases
      parentheses are required as part of the fixed syntax of a
      particular SQL command.
     
      Brackets ([]) are used to select the elements
      of an array.  See Section 8.15 for more information
      on arrays.
     
      Commas (,) are used in some syntactical
      constructs to separate the elements of a list.
     
      The semicolon (;) terminates an SQL command.
      It cannot appear anywhere within a command, except within a
      string constant or quoted identifier.
     
      The colon (:) is used to select
      “slices” from arrays. (See Section 8.15.)  In certain SQL dialects (such as Embedded
      SQL), the colon is used to prefix variable names.
     
      The asterisk (*) is used in some contexts to denote
      all the fields of a table row or composite value.  It also
      has a special meaning when used as the argument of an
      aggregate function, namely that the aggregate does not require
      any explicit parameter.
     
      The period (.) is used in numeric
      constants, and to separate schema, table, and column names.
     
A comment is a sequence of characters beginning with double dashes and extending to the end of the line, e.g.:
-- This is a standard SQL comment
Alternatively, C-style block comments can be used:
/* multiline comment * with nesting: /* nested block comment */ */
    where the comment begins with /* and extends to
    the matching occurrence of */. These block
    comments nest, as specified in the SQL standard but unlike C, so that one can
    comment out larger blocks of code that might contain existing block
    comments.
   
A comment is removed from the input stream before further syntax analysis and is effectively replaced by whitespace.
Table 4.2 shows the precedence and associativity of the operators in PostgreSQL. Most operators have the same precedence and are left-associative. The precedence and associativity of the operators is hard-wired into the parser.
You will sometimes need to add parentheses when using combinations of binary and unary operators. For instance:
SELECT 5 ! - 6;
will be parsed as:
SELECT 5 ! (- 6);
    because the parser has no idea — until it is too late
    — that ! is defined as a postfix operator,
    not an infix one.  To get the desired behavior in this case, you
    must write:
SELECT (5 !) - 6;
This is the price one pays for extensibility.
Table 4.2. Operator Precedence (highest to lowest)
| Operator/Element | Associativity | Description | 
|---|---|---|
| . | left | table/column name separator | 
| :: | left | PostgreSQL-style typecast | 
| [] | left | array element selection | 
| +- | right | unary plus, unary minus | 
| ^ | left | exponentiation | 
| */% | left | multiplication, division, modulo | 
| +- | left | addition, subtraction | 
| (any other operator) | left | all other native and user-defined operators | 
| BETWEENINLIKEILIKESIMILAR | range containment, set membership, string matching | |
| <>=<=>=<> | comparison operators | |
| ISISNULLNOTNULL | IS TRUE,IS FALSE,IS
       NULL,IS DISTINCT FROM, etc | |
| NOT | right | logical negation | 
| AND | left | logical conjunction | 
| OR | left | logical disjunction | 
Note that the operator precedence rules also apply to user-defined operators that have the same names as the built-in operators mentioned above. For example, if you define a “+” operator for some custom data type it will have the same precedence as the built-in “+” operator, no matter what yours does.
    When a schema-qualified operator name is used in the
    OPERATOR syntax, as for example in:
SELECT 3 OPERATOR(pg_catalog.+) 4;
    the OPERATOR construct is taken to have the default precedence
    shown in Table 4.2 for
    “any other operator”.  This is true no matter
    which specific operator appears inside OPERATOR().
   
     PostgreSQL versions before 9.5 used slightly different
     operator precedence rules.  In particular, <=
     >= and <> used to be treated as
     generic operators; IS tests used to have higher priority;
     and NOT BETWEEN and related constructs acted inconsistently,
     being taken in some cases as having the precedence of NOT
     rather than BETWEEN.  These rules were changed for better
     compliance with the SQL standard and to reduce confusion from
     inconsistent treatment of logically equivalent constructs.  In most
     cases, these changes will result in no behavioral change, or perhaps
     in “no such operator” failures which can be resolved by adding
     parentheses.  However there are corner cases in which a query might
     change behavior without any parsing error being reported.  If you are
     concerned about whether these changes have silently broken something,
     you can test your application with the configuration
     parameter operator_precedence_warning turned on
     to see if any warnings are logged.