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select(3)





NAME

       SELECT - retrieve rows from a table or view


SYNOPSIS

       SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
           * | expression [ AS output_name ] [, ...]
           [ FROM from_item [, ...] ]
           [ WHERE condition ]
           [ GROUP BY expression [, ...] ]
           [ HAVING condition [, ...] ]
           [ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]
           [ ORDER BY expression [ ASC | DESC | USING operator ] [, ...] ]
           [ LIMIT { count | ALL } ]
           [ OFFSET start ]
           [ FOR { UPDATE | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] [...] ]

       where from_item can be one of:

           [ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
           ( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
           function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
           function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
           from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]


DESCRIPTION

       SELECT retrieves rows from zero or more tables.  The general processing
       of SELECT is as follows:

       1.     All elements in the FROM list are computed.   (Each  element  in
              the FROM list is a real or virtual table.) If more than one ele-
              ment is specified  in  the  FROM  list,  they  are  cross-joined
              together.  (See FROM Clause [select(5)] below.)

       2.     If  the  WHERE clause is specified, all rows that do not satisfy
              the condition are eliminated from the output. (See WHERE  Clause
              [select(5)] below.)

       3.     If  the GROUP BY clause is specified, the output is divided into
              groups of rows that match on one or more values. If  the  HAVING
              clause  is present, it eliminates groups that do not satisfy the
              given condition. (See GROUP BY  Clause  [select(5)]  and  HAVING
              Clause [select(5)] below.)

       4.     The  actual  output  rows  are  computed using the SELECT output
              expressions for each selected row. (See SELECT List  [select(5)]
              below.)

       5.     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.  (See  UNION  Clause  [select(5)],  INTERSECT
              Clause [select(l)], and EXCEPT Clause [select(5)] below.)

       6.     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 [select(5)] below.)

       7.     DISTINCT eliminates duplicate rows from the result. DISTINCT  ON
              eliminates rows that match on all the specified expressions. ALL
              (the default) will return all candidate rows,  including  dupli-
              cates. (See DISTINCT Clause [select(5)] below.)

       8.     If the LIMIT or OFFSET clause is specified, the SELECT statement
              only returns a subset of the  result  rows.  (See  LIMIT  Clause
              [select(5)] below.)

       9.     If  FOR  UPDATE  or FOR SHARE is specified, the SELECT statement
              locks the selected rows against  concurrent  updates.  (See  FOR
              UPDATE/FOR SHARE Clause [select(5)] below.)

       You  must  have SELECT privilege on a table to read its values. The use
       of FOR UPDATE or FOR SHARE requires UPDATE privilege as well.


PARAMETERS

   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 to restrict the returned rows to a small subset of the Carte-
       sian 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, only that table is scanned. If ONLY
              is not specified, the table and all its  descendant  tables  (if
              any)  are  scanned. * can be appended to the table name to indi-
              cate that descendant tables are to be scanned, but in  the  cur-
              rent  version, this is the default behavior. (In releases before
              7.1, ONLY was the default behavior.) The default behavior can be
              modified by changing the sql_inheritance configuration option.

       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 remain-
              der 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.

       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 [values(5)] command can also be used here.

       function_name
              Function calls can appear in the FROM  clause.  (This  is  espe-
              cially  useful  for  functions  that return result sets, but any
              function can be used.) This acts as though its output were  cre-
              ated as a temporary table for the duration of this single SELECT
              command. An alias may also be used. 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.  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 ( col-
              umn_name data_type [, ... ] ). The column definition  list  must
              match  the  actual  number  and types of columns returned by the
              function.

       join_type
              One of

              o [ INNER ] JOIN

              o LEFT [ OUTER ] JOIN

              o RIGHT [ OUTER ] JOIN

              o FULL [ OUTER ] JOIN

              o CROSS JOIN

       For the INNER and OUTER join types, a join condition must be specified,
       namely exactly one of NATURAL, ON join_condition, or USING (join_column
       [, ...]).  See below for the meaning. For CROSS  JOIN,  none  of  these
       clauses may appear.

       A  JOIN clause combines two FROM items. 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 items.

       CROSS JOIN and INNER JOIN produce a simple Cartesian product, the  same
       result  as you get from listing the two items at the top level of FROM,
       but restricted by the join condition (if any).  CROSS JOIN  is  equiva-
       lent  to INNER JOIN ON (TRUE), that is, no rows are removed by qualifi-
       cation.  These join types 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 inputs.

       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_condition
              join_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 [, ...])
              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  equiva-
              lent columns will be included in the join output, not both.

       NATURAL
              NATURAL  is shorthand for a USING list that mentions all columns
              in the two tables that have the same names.

   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 elimi-
       nated 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 expression [, ...]

       GROUP  BY  will condense into a single row all selected rows that share
       the same values for the grouped expressions. expression 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.

       Aggregate functions, if any are used, are computed across all rows mak-
       ing  up  each group, producing a separate value for each group (whereas
       without GROUP BY, an aggregate produces a single value computed  across
       all  the selected rows).  When GROUP BY is present, it is not valid for
       the SELECT list expressions to refer to ungrouped columns except within
       aggregate  functions, since there would be more than one possible value
       to return for an ungrouped column.

   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 refer-
       ence a grouping column, unless the reference appears within  an  aggre-
       gate function.

       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.

   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. Using the clause AS output_name, another name can be  specified
       for  an  output column. This name is primarily used to label the column
       for display. It can also 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, one can write
       table_name.* as a shorthand for the columns coming from just  that  ta-
       ble.

   UNION CLAUSE
       The UNION clause has this general form:

       select_statement UNION [ ALL ] select_statement

       select_statement  is  any  SELECT statement without an ORDER BY, LIMIT,
       FOR UPDATE, or FOR SHARE clause.  (ORDER BY and LIMIT can  be  attached
       to  a subexpression if it is enclosed in parentheses. Without parenthe-
       ses, 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 com-
       patible data types.

       The result of UNION does not contain any duplicate rows unless the  ALL
       option  is  specified.  ALL prevents elimination of duplicates. (There-
       fore, UNION ALL is usually significantly quicker than  UNION;  use  ALL
       when you can.)

       Multiple  UNION  operators  in  the same SELECT statement are evaluated
       left to right, unless otherwise indicated by parentheses.

       Currently, FOR UPDATE and FOR SHARE may not be specified either  for  a
       UNION result or for any input of a UNION.

   INTERSECT CLAUSE
       The INTERSECT clause has this general form:

       select_statement INTERSECT [ ALL ] select_statement

       select_statement  is  any  SELECT statement without an ORDER BY, LIMIT,
       FOR UPDATE, or FOR SHARE clause.

       The INTERSECT operator  computes  the  set  intersection  of  the  rows
       returned  by  the involved SELECT statements. A row is in the intersec-
       tion 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.

       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 UPDATE and FOR SHARE may not be specified either for  an
       INTERSECT result or for any input of an INTERSECT.

   EXCEPT CLAUSE
       The EXCEPT clause has this general form:

       select_statement EXCEPT [ ALL ] select_statement

       select_statement  is  any  SELECT statement without an ORDER BY, LIMIT,
       FOR UPDATE, or FOR 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.

       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 UPDATE and FOR SHARE may not 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 BY expression [ ASC | DESC | USING operator ] [, ...]

       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  ORDER  BY  clause causes the result rows to be sorted according to
       the specified expressions. If two rows are equal according to the left-
       most  expression, the 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.

       The  ordinal  number  refers to the ordinal (left-to-right) position of
       the result 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 a result 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 result 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 may only specify an out-
       put column name or number, not an expression.

       If  an  ORDER BY expression is a simple name that matches both a result
       column name and an input column name, ORDER BY will interpret it as the
       result  column  name.  This is the opposite of the choice that GROUP BY
       will make in the same situation. This inconsistency is made to be  com-
       patible with the SQL standard.

       Optionally  one  may add the key word ASC (ascending) or DESC (descend-
       ing) after any expression in the ORDER BY clause. If not specified, ASC
       is assumed by default. Alternatively, a specific ordering operator name
       may be specified in the USING clause.  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.)

       The  null value sorts higher than any other value. In other words, with
       ascending sort order, null values sort at the end, and with  descending
       sort order, null values sort at the beginning.

       Character-string data is sorted according to the locale-specific colla-
       tion order that was established when the database cluster was  initial-
       ized.

   DISTINCT CLAUSE
       If  DISTINCT  is  specified,  all  duplicate  rows are removed from the
       result set (one row is kept from each group of duplicates). ALL  speci-
       fies the opposite: all rows are kept; that is the default.

       DISTINCT ON ( expression [, ...] ) keeps only the first row of each set
       of rows where the given expressions evaluate to equal. The 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 expres-
       sion(s). The ORDER BY clause will normally contain  additional  expres-
       sion(s)  that determine the desired precedence of rows within each DIS-
       TINCT ON group.

   LIMIT CLAUSE
       The LIMIT clause consists of two independent sub-clauses:

       LIMIT { count | ALL }
       OFFSET start

       count specifies the maximum number of rows to return, while start spec-
       ifies  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.

       When using LIMIT, it is a good idea to use an ORDER BY clause that con-
       strains the result rows into a unique order. Otherwise you will get  an
       unpredictable  subset  of the query's rows -- you may 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.

   FOR UPDATE/FOR SHARE CLAUSE
       The FOR UPDATE clause has this form:

       FOR UPDATE [ OF table_name [, ...] ] [ NOWAIT ]

       The closely related FOR SHARE clause has this form:

       FOR SHARE [ OF table_name [, ...] ] [ NOWAIT ]

       FOR  UPDATE  causes  the  rows  retrieved by the SELECT statement to be
       locked as though for update. This prevents them from being modified  or
       deleted  by other transactions until the current transaction ends. That
       is, other transactions that  attempt  UPDATE,  DELETE,  or  SELECT  FOR
       UPDATE  of  these  rows  will  be blocked until the current transaction
       ends.  Also, if an UPDATE, DELETE, or SELECT FOR  UPDATE  from  another
       transaction  has  already  locked  a  selected  row or rows, SELECT FOR
       UPDATE will wait for the other transaction to complete, and  will  then
       lock  and  return  the updated row (or no row, if the row was deleted).
       For further discussion see in the documentation.

       To prevent the operation from waiting for other transactions to commit,
       use  the  NOWAIT  option.  SELECT  FOR  UPDATE NOWAIT reports an error,
       rather than waiting, if a selected row cannot  be  locked  immediately.
       Note  that NOWAIT applies only to the row-level lock(s) -- the required
       ROW SHARE table-level lock is still taken in the ordinary way  (see  in
       the  documentation). You can use the NOWAIT option of LOCK [lock(5)] if
       you need to acquire the table-level lock without waiting.

       FOR SHARE behaves similarly, except that it acquires  a  shared  rather
       than  exclusive  lock on each retrieved row. A shared lock blocks other
       transactions from performing UPDATE, DELETE, or SELECT  FOR  UPDATE  on
       these  rows,  but  it  does not prevent them from performing SELECT FOR
       SHARE.

       If specific tables are named in FOR UPDATE or FOR SHARE, then only rows
       coming  from  those  tables  are  locked;  any other tables used in the
       SELECT are simply read as usual. A FOR UPDATE or FOR SHARE clause with-
       out a table list affects all tables used in the command.  If FOR UPDATE
       or FOR SHARE is applied to a view or sub-query, it affects  all  tables
       used in the view or sub-query.

       Multiple  FOR UPDATE and FOR SHARE clauses can be written if it is nec-
       essary to specify different locking behavior for different  tables.  If
       the same table is mentioned (or implicitly affected) by both FOR UPDATE
       and FOR SHARE clauses, then it is processed as FOR UPDATE. Similarly, a
       table is processed as NOWAIT if that is specified in any of the clauses
       affecting it.

       FOR UPDATE and FOR SHARE cannot be used in contexts where returned rows
       can't  be  clearly  identified  with individual table rows; for example
       they can't be used with aggregation.

              Caution: Avoid locking a row and  then  modifying  it  within  a
              later  savepoint or PL/pgSQL exception block. A subsequent roll-
              back would cause the lock to be lost. For example,

              BEGIN;
              SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
              SAVEPOINT s;
              UPDATE mytable SET ... WHERE key = 1;
              ROLLBACK TO s;

              After the ROLLBACK, the row is effectively unlocked, rather than
              returned to its pre-savepoint state of being locked but not mod-
              ified.  This hazard occurs if a row locked in the current trans-
              action is updated or deleted, or if a shared lock is upgraded to
              exclusive: in all these cases, the former lock state is  forgot-
              ten.  If  the transaction is then rolled back to a state between
              the original locking command and the subsequent change, the  row
              will  appear  not to be locked at all. This is an implementation
              deficiency which will be addressed in a future release of  Post-
              greSQL.

              Caution:  It  is  possible for a SELECT command using both LIMIT
              and FOR UPDATE/SHARE clauses to return fewer rows than specified
              by  LIMIT.   This is because LIMIT is applied first. The command
              selects the specified number of rows, but might then block  try-
              ing  to  obtain  lock  on  one or more of them.  Once the SELECT
              unblocks, the row might have been deleted or updated so that  it
              does  not  meet the query WHERE condition anymore, in which case
              it will not be returned.


EXAMPLES

       To join the table films with the table distributors:

       SELECT f.title, f.did, d.name, f.date_prod, f.kind
           FROM distributors d, films f
           WHERE f.did = d.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  distribu-
       tors  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


COMPATIBILITY

       Of course, the SELECT statement is compatible with  the  SQL  standard.
       But there are some extensions and some missing features.

   OMITTED 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.

       Note that if a FROM clause is not specified, the query cannot reference
       any database tables. For example, the following query is invalid:

       SELECT distributors.* WHERE distributors.name = 'Westward';

       PostgreSQL releases prior to 8.1 would accept queries of this form, and
       add  an implicit entry to the query's FROM clause for each table refer-
       enced by the query. This is no longer the default behavior, because  it
       does  not comply with the SQL standard, and is considered by many to be
       error-prone. For compatibility with  applications  that  rely  on  this
       behavior the add_missing_from configuration variable can be enabled.

   THE AS KEY WORD
       In  the SQL standard, the optional key word AS is just noise and can be
       omitted without affecting the meaning. The PostgreSQL  parser  requires
       this key word when renaming output columns because the type extensibil-
       ity features lead to parsing ambiguities without it.  AS is optional in
       FROM items, however.

   NAMESPACE AVAILABLE TO GROUP BY AND ORDER BY
       In  the  SQL-92 standard, an ORDER BY clause may only use result column
       names or numbers, while a GROUP BY  clause  may  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 interpre-
       tation  if there is ambiguity).  PostgreSQL also allows both clauses to
       specify arbitrary expressions. Note that names appearing in an  expres-
       sion  will  always be taken as input-column names, not as result-column
       names.

       SQL:1999 and later use a slightly different  definition  which  is  not
       entirely  upward compatible with SQL-92.  In most cases, however, Post-
       greSQL will interpret an ORDER BY or GROUP BY expression the  same  way
       SQL:1999 does.

   NONSTANDARD CLAUSES
       The  clauses  DISTINCT ON, LIMIT, and OFFSET are not defined in the SQL
       standard.

SQL - Language Statements         2008-06-08                          SELECT()

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