Provided by: postgresql-client-8.0_8.0.7-2build1_i386 bug

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 [ OF table_name [, ...] ] ]

       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 one 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
              element is specified in the FROM  list,  they  are  cross-joined
              together.  (See FROM Clause [select(7)] 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(7)] 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(7)]  and  HAVING
              Clause [select(7)] below.)

       4.     The  actual  output  rows  are  computed using the SELECT output
              expressions for each selected row. (See SELECT List  [select(7)]
              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(7)],  INTERSECT
              Clause [select(l)], and EXCEPT Clause [select(7)] 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(7)] 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
              duplicates. (See DISTINCT Clause [select(7)] 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(7)] below.)

       9.     The  FOR  UPDATE  clause causes the SELECT statement to lock the
              selected rows against concurrent updates. (See FOR UPDATE Clause
              [select(7)] below.)

       You  must  have SELECT privilege on a table to read its values. The use
       of FOR UPDATE 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
       Cartesian product.

       The FROM clause can contain the following elements:

       table_name
              The name (optionally schema-qualified) of an existing  table  or
              view.  If ONLY is specified, 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
              indicate that descendant tables are to be scanned,  but  in  the
              current  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
              remainder  of  the  SELECT must refer to this FROM item as f not
              foo. If an alias is written, a column alias  list  can  also  be
              written  to  provide substitute names for one or more columns of
              the table.

       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
              especially useful for functions that return result sets, but any
              function can be used.) This  acts  as  though  its  output  were
              created  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 ( column_name data_type [, ... ] ). The  column  definition
              list  must match the actual number and types of columns returned
              by the function.

       join_type
              One of

              · [ INNER ] JOIN

              · LEFT [ OUTER ] JOIN

              · RIGHT [ OUTER ] JOIN

              · FULL [ OUTER ] JOIN

              · 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
       equivalent  to  INNER  JOIN  ON (TRUE), that is, no rows are removed by
       qualification.  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
              equivalent  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
       eliminated from the output. A row satisfies the condition if it returns
       true when the actual  row  values  are  substituted  for  any  variable
       references.

   GROUP BY CLAUSE
       The optional GROUP BY clause has the general form

       GROUP BY 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
       making  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
       reference a grouping column, unless the  reference  appears  within  an
       aggregate function.

   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
       table.

   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, or
       FOR  UPDATE  clause.   (ORDER  BY  and  LIMIT  can  be  attached  to  a
       subexpression  if  it  is enclosed in parentheses. Without parentheses,
       these clauses will be taken to apply to the result of the UNION, not to
       its right-hand input expression.)

       The  UNION  operator computes the set union of the rows returned by the
       involved SELECT statements. A row is in the set  union  of  two  result
       sets  if  it appears in at least one of the result sets. The two SELECT
       statements that represent the direct operands of the UNION must produce
       the  same  number  of  columns,  and  corresponding  columns must be of
       compatible data types.

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

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

       Currently, FOR UPDATE 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, or
       FOR UPDATE clause.

       The INTERSECT operator  computes  the  set  intersection  of  the  rows
       returned   by   the  involved  SELECT  statements.  A  row  is  in  the
       intersection of two result sets if it appears in both result sets.

       The result of INTERSECT does not contain any duplicate rows unless  the
       ALL  option is specified.  With ALL, a row that has m duplicates in the
       left table and n duplicates in the right  table  will  appear  min(m,n)
       times in the result set.

       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 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, or
       FOR UPDATE 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 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
       leftmost  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
       output 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
       compatible with the SQL standard.

       Optionally   one   may  add  the  key  word  ASC  (ascending)  or  DESC
       (descending) after any expression  in  the  ORDER  BY  clause.  If  not
       specified,  ASC  is  assumed  by  default.  Alternatively,  a  specific
       ordering operator name 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
       collation  order  that  was  established  when the database cluster was
       initialized.

   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
       specifies 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
       expression(s).  The  ORDER  BY  clause will normally contain additional
       expression(s) that determine the desired precedence of rows within each
       DISTINCT 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
       specifies the number of rows to skip before starting  to  return  rows.
       When  both  are  specified,  start  rows are skipped before starting to
       count the count rows to be returned.

       When using LIMIT, it is a good idea to use  an  ORDER  BY  clause  that
       constrains  the result rows into a unique order. Otherwise you will get
       an unpredictable subset of the query’s rows — you 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 CLAUSE
       The FOR UPDATE clause has this form:

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

       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.

       If  specific tables are named in FOR UPDATE, then only rows coming from
       those tables are locked; any other tables used in the SELECT are simply
       read as usual.

       FOR  UPDATE  cannot  be  used  in contexts where returned rows can’t be
       clearly identified with individual table rows; for example it can’t  be
       used with aggregation.

       FOR  UPDATE  may  appear before LIMIT for compatibility with PostgreSQL
       versions before 7.3. It effectively executes after LIMIT, however,  and
       so that is the recommended place to write it.

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
       distributors and actors, restricting the results to  those  that  begin
       with  the letter W in each table. Only distinct rows are wanted, so the
       key word ALL is omitted.

       distributors:               actors:
        did |     name              id |     name
       -----+--------------        ----+----------------
        108 | Westward               1 | Woody Allen
        111 | Walt Disney            2 | Warren Beatty
        112 | Warner Bros.           3 | Walter Matthau
        ...                         ...

       SELECT distributors.name
           FROM distributors
           WHERE distributors.name LIKE ’W%’
       UNION
       SELECT actors.name
           FROM actors
           WHERE actors.name LIKE ’W%’;

             name
       ----------------
        Walt Disney
        Walter Matthau
        Warner Bros.
        Warren Beatty
        Westward
        Woody Allen

       This example shows how to use a function in the FROM clause, both  with
       and without a column definition list:

       CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
           SELECT * FROM distributors WHERE did = $1;
       $$ LANGUAGE SQL;

       SELECT * FROM distributors(111);
        did |    name
       -----+-------------
        111 | Walt Disney

       CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
           SELECT * FROM distributors WHERE did = $1;
       $$ LANGUAGE SQL;

       SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
        f1  |     f2
       -----+-------------
        111 | Walt Disney

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.

       A less obvious use is to abbreviate a normal SELECT from tables:

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

        did |   name
       -----+----------
        108 | Westward

       This works because an implicit FROM item is added for each  table  that
       is  referenced in other parts of the SELECT statement but not mentioned
       in FROM.

       While this is a convenient shorthand, it’s easy to misuse. For example,
       the command

       SELECT distributors.* FROM distributors d;

       is probably a mistake; most likely the user meant

       SELECT d.* FROM distributors d;

       rather than the unconstrained join

       SELECT distributors.* FROM distributors d, distributors distributors;

       that  he  will  actually  get.  To  help  detect  this sort of mistake,
       PostgreSQL will warn if the implicit-FROM feature is used in  a  SELECT
       statement  that  also  contains  an  explicit  FROM clause. Also, it is
       possible  to  disable  the  implicit-FROM  feature   by   setting   the
       add_missing_from parameter to false.

   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
       extensibility 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
       interpretation  if  there  is  ambiguity).  PostgreSQL also allows both
       clauses to specify arbitrary expressions. Note that names appearing  in
       an  expression  will  always  be  taken  as  input-column names, not as
       result-column names.

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

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