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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(l)] 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(l)] 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(l)]  and  HAVING
              Clause [select(l)] below.)

       4.     The  actual  output  rows  are  computed using the SELECT output
              expressions for each selected row. (See SELECT List  [select(l)]
              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(l)],  INTERSECT
              Clause [select(l)], and EXCEPT Clause [select(l)] 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(l)] 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(l)] 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(l)] below.)

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

       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 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 the
       documentation). You can use the NOWAIT option of LOCK [lock(l)] 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.

       It  is  currently  not allowed for a single SELECT statement to include
       both FOR UPDATE and FOR SHARE, nor can different parts of the statement
       use both NOWAIT and normal waiting mode.

       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.

       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.

       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 speci-
       fied number of rows, but might then block trying 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         2005-11-05                          SELECT()

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