Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.
PQexec
          Submit a command to the server
          and wait for the result.
PGresult *PQexec(PGconn *conn,
                 const char *query);
          Returns a PGresult pointer or possibly a NULL pointer.
          A non-NULL pointer will generally be returned except in
          out-of-memory conditions or serious errors such as inability
          to send the command to the backend.
          If a NULL is returned, it
	  should be treated like a PGRES_FATAL_ERROR result.  Use
	  PQerrorMessage to get more information about the error.
The PGresult structure encapsulates the result
returned by the backend.
libpq application programmers should be careful to
maintain the PGresult abstraction.  Use the accessor functions below to get
at the contents of PGresult.  Avoid directly referencing the fields of the
PGresult structure because they are subject to change in the future.
(Beginning in PostgreSQL 6.4, the
definition of struct PGresult is not even provided in libpq-fe.h.  If you
have old code that accesses PGresult fields directly, you can keep using it
by including libpq-int.h too, but you are encouraged to fix the code
soon.)
PQresultStatus
          Returns the result status of the command.
ExecStatusType PQresultStatus(const PGresult *res)
PQresultStatus can return one of the following values:
PGRES_EMPTY_QUERY -- The string sent to the backend was empty.
PGRES_COMMAND_OK -- Successful completion of a command returning no data
PGRES_TUPLES_OK -- The query successfully executed
PGRES_COPY_OUT -- Copy Out (from server) data transfer started
PGRES_COPY_IN -- Copy In (to server) data transfer started
PGRES_BAD_RESPONSE -- The server's response was not understood
PGRES_NONFATAL_ERROR
PGRES_FATAL_ERROR
If the result status is PGRES_TUPLES_OK, then the routines described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT, UPDATE, etc.). A response of PGRES_EMPTY_QUERY often exposes a bug in the client software.
PQresStatus
	Converts the enumerated type returned by PQresultStatus into
	a string constant describing the status code.
char *PQresStatus(ExecStatusType status);
PQresultErrorMessage
returns the error message associated with the query, or an empty string
if there was no error.
char *PQresultErrorMessage(const PGresult *res);
Immediately following a PQexec or PQgetResult
call, PQerrorMessage (on the connection) will return the same
string as PQresultErrorMessage (on the result).  However, a
PGresult will retain its error message
until destroyed, whereas the connection's error message will change when
subsequent operations are done.  Use PQresultErrorMessage when you want to
know the status associated with a particular PGresult; use PQerrorMessage
when you want to know the status from the latest operation on the connection.
PQclear
          Frees  the  storage  associated with the PGresult.
          Every query result should be freed via PQclear when
          it  is  no  longer needed.
void PQclear(PQresult *res);
          You can keep a PGresult object around for as long as you
          need it; it does not go away when you issue a new query,
          nor even if you close the connection.  To get rid of it,
          you must call PQclear.  Failure to do this will
          result in memory leaks in  the  frontend  application.
PQmakeEmptyPGresult
          Constructs an empty PGresult object with the given status.
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
This is libpq's internal routine to allocate and initialize an empty
PGresult object.  It is exported because some applications find it
useful to generate result objects (particularly objects with error
status) themselves.  If conn is not NULL and status indicates an error,
the connection's current errorMessage is copied into the PGresult.
Note that PQclear should eventually be called on the object, just
as with a PGresult returned by libpq itself.
PQescapeString
          Escapes a string for use within an SQL query.
size_t PQescapeString (char *to, const char *from, size_t length);
If you want to include strings that have been received from a source that is not trustworthy (for example, because a random user entered them), you cannot directly include them in SQL queries for security reasons. Instead, you have to quote special characters that are otherwise interpreted by the SQL parser.
PQescapeString performs this operation.  The
from points to the first character of the string that
is to be escaped, and the length parameter counts the
number of characters in this string (a terminating zero byte is
neither necessary nor counted).  to shall point to a
buffer that is able to hold at least one more character than twice
the value of length, otherwise the behavior is
undefined.  A call to PQescapeString writes an escaped
version of the from string to the to
buffer, replacing special characters so that they cannot cause any
harm, and adding a terminating zero byte.  The single quotes that
must surround PostgreSQL string literals are not part of the result
string.
PQescapeString returns the number of characters written
to to, not including the terminating zero byte.
Behavior is undefined when the to and from
strings overlap.
   PQescapeBytea
   Escapes a binary string (bytea type) for use within an SQL query.
   
    unsigned char *PQescapeBytea(unsigned char *from,
                                         size_t from_length,
                                         size_t *to_length);
   
   Certain ASCII characters must
   be escaped (but all characters may be escaped)
   when used as part of a bytea
   string literal in an SQL statement. In general, to
   escape a character, it is converted into the three digit octal number
   equal to the decimal ASCII value, and preceded by
   two backslashes. The single quote (') and backslash (\) characters have
   special alternate escape sequences. See the User's Guide
   for more information. PQescapeBytea
    performs this operation, escaping only the minimally
   required characters.
  
   The from parameter points to the first
   character of the string that is to be escaped, and the
   from_length parameter reflects the number of
   characters in this binary string (a terminating zero byte is
   neither necessary nor counted).  The to_length
   parameter shall point to a buffer suitable to hold the resultant
   escaped string length. The result string length does not
   include the terminating zero byte of the result.
  
   PQescapeBytea returns an escaped version of the
   from parameter binary string, to a caller-provided
    buffer. The return string has all special characters replaced
   so that they can be properly processed by the PostgreSQL string literal
   parser, and the bytea input function. A terminating zero
   byte is also added.  The single quotes that must surround
   PostgreSQL string literals are not part of the result string.
  
PQntuples
          Returns the number of tuples (rows)
          in the query result.
int PQntuples(const PGresult *res);
PQnfields
          Returns   the   number    of    fields
          (columns) in each row of the query result.
int PQnfields(const PGresult *res);
PQfname
 Returns the field (column) name associated with the given field index.
 Field  indices start at 0.
char *PQfname(const PGresult *res,
                    int field_index);PQfnumber
            Returns  the  field  (column)  index
          associated with the given field name.
int PQfnumber(const PGresult *res,
              const char *field_name);-1 is returned if the given name does not match any field.
PQftype
            Returns the field type associated with the
          given  field  index.  The  integer  returned is an
          internal coding of the type.  Field indices  start
          at 0.
Oid PQftype(const PGresult *res,
            int field_index);You can query the system table pg_type to obtain the name and properties of the various data types. The OIDs of the built-in data types are defined in src/include/catalog/pg_type.h in the source tree.
PQfmod
          Returns  the type-specific modification data of the field
          associated with the given field index.
          Field indices start at 0.
int PQfmod(const PGresult *res,
           int field_index);PQfsize
          Returns  the  size  in bytes of the field
          associated with the given field index.
          Field indices start at 0.
int PQfsize(const PGresult *res,
            int field_index);
	PQfsize returns the space allocated for this field in a database
	tuple, in other words the size of the server's binary representation
	of the data type.  -1 is returned if the field is variable size.
PQbinaryTuples
          Returns 1 if the PGresult contains binary tuple data,
	  0 if it contains ASCII data.
int PQbinaryTuples(const PGresult *res);
Currently, binary tuple data can only be returned by a query that extracts data from a binary cursor.
PQgetvalue
            Returns a single field  (column)  value of one tuple (row)
	    of a PGresult.
	    Tuple and field indices start at 0.
char* PQgetvalue(const PGresult *res,
                 int tup_num,
                 int field_num);
For most queries, the value returned by PQgetvalue
is a null-terminated character string  representation
of the attribute value.  But if PQbinaryTuples() is 1,
the  value  returned  by PQgetvalue  is  the  binary
representation of the
type in the internal format of the backend server
(but not including the size word, if the field is variable-length).
It  is then the programmer's responsibility to cast and
convert the data to the correct C type.  The pointer
returned  by  PQgetvalue points to storage that is
part of the PGresult structure.  One should not modify it,
and one must explicitly 
copy the value into other storage if it is to
be used past the lifetime of the  PGresult  structure itself.
PQgetisnull
           Tests a field for a NULL entry.
           Tuple and field indices start at 0.
int PQgetisnull(const PGresult *res,
                int tup_num,
                int field_num);
            This function returns  1 if the field contains a NULL, 0 if
            it contains a non-null value.  (Note that PQgetvalue
            will return an empty string, not a null pointer, for a NULL
            field.)
PQgetlength
          Returns   the   length  of  a  field (attribute) value in bytes.
          Tuple and field indices start at 0.
int PQgetlength(const PGresult *res,
                int tup_num,
                int field_num);
This is the actual data length for the particular data value, that is the
size of the object pointed to by PQgetvalue.  Note that for character-represented
values, this size has little to do with the binary size reported by PQfsize.
PQprint
          Prints out all the  tuples  and,  optionally,  the
          attribute  names  to  the specified output stream.
       
void PQprint(FILE* fout,      /* output stream */
             const PGresult *res,
             const PQprintOpt *po);
struct {
    pqbool  header;      /* print output field headings and row count */
    pqbool  align;       /* fill align the fields */
    pqbool  standard;    /* old brain dead format */
    pqbool  html3;       /* output html tables */
    pqbool  expanded;    /* expand tables */
    pqbool  pager;       /* use pager for output if needed */
    char    *fieldSep;   /* field separator */
    char    *tableOpt;   /* insert to HTML table ... */
    char    *caption;    /* HTML caption */
    char    **fieldName; /* null terminated array of replacement field names */
} PQprintOpt;
       This function was formerly used by psql to print query results, but this is no longer the case and this function is no longer actively supported.
PQcmdStatus
          Returns the command status string from the SQL command that
	  generated the PGresult.
char * PQcmdStatus(const PGresult *res);
PQcmdTuples
	  Returns the number of rows affected by the SQL command.
char * PQcmdTuples(const PGresult *res);
If the SQL command that generated the PGresult was INSERT, UPDATE or DELETE, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string.
PQoidValue
          Returns the object ID of the inserted row, if the
	  SQL command was an INSERT
	  that inserted exactly one row into a table that has OIDs.
          Otherwise, returns InvalidOid.
Oid PQoidValue(const PGresult *res);
The type Oid and the constant InvalidOid will be defined if you include the libpq header file. They will both be some integer type.
PQoidStatus
          Returns a string with the object ID of the inserted row, if the
	  SQL command was an INSERT.
	  (The string will be 0 if the INSERT did not insert exactly one
	  row, or if the target table does not have OIDs.)  If the command
	  was not an INSERT, returns an empty string.
char * PQoidStatus(const PGresult *res);
This function is deprecated in favor of PQoidValue
and is not thread-safe.