SELECT(7)		PostgreSQL 9.2.24 Documentation		     SELECT(7)



NAME
       SELECT, TABLE, WITH - retrieve rows from a table or view

SYNOPSIS
       [ WITH [ RECURSIVE ] with_query [, ...] ]
       SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
	   * | expression [ [ AS ] output_name ] [, ...]
	   [ FROM from_item [, ...] ]
	   [ WHERE condition ]
	   [ GROUP BY expression [, ...] ]
	   [ HAVING condition [, ...] ]
	   [ WINDOW window_name AS ( window_definition ) [, ...] ]
	   [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]
	   [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
	   [ LIMIT { count | ALL } ]
	   [ OFFSET start [ ROW | ROWS ] ]
	   [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ]
	   [ 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 [, ...] ) ]
	   with_query_name [ [ 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 [, ...] ) ]

       and with_query is:

	   with_query_name [ ( column_name [, ...] ) ] AS ( select | values | insert | update | delete )

       TABLE [ ONLY ] table_name [ * ]

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

	1. All queries in the WITH list are computed. These effectively serve
	   as temporary tables that can be referenced in the FROM list. A WITH
	   query that is referenced more than once in FROM is computed only
	   once. (See WITH Clause below.)

	2. 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 below.)

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

	4. If the GROUP BY clause is specified, the output is combined 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 and HAVING Clause below.)

	5. The actual output rows are computed using the SELECT output
	   expressions for each selected row or row group. (See SELECT List
	   below.)

	6. SELECT DISTINCT eliminates duplicate rows from the result.  SELECT
	   DISTINCT ON eliminates rows that match on all the specified
	   expressions.	 SELECT ALL (the default) will return all candidate
	   rows, including duplicates. (See DISTINCT Clause below.)

	7. 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. The noise word DISTINCT can be added to explicitly
	   specify eliminating duplicate rows. Notice that DISTINCT is the
	   default behavior here, even though ALL is the default for SELECT
	   itself. (See UNION Clause, INTERSECT Clause, and EXCEPT Clause
	   below.)

	8. 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 below.)

	9. If the LIMIT (or FETCH FIRST) or OFFSET clause is specified, the
	   SELECT statement only returns a subset of the result rows. (See
	   LIMIT Clause below.)

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

       You must have SELECT privilege on each column used in a SELECT command.
       The use of FOR UPDATE or FOR SHARE requires UPDATE privilege as well
       (for at least one column of each table so selected).

PARAMETERS
   WITH Clause
       The WITH clause allows you to specify one or more subqueries that can
       be referenced by name in the primary query. The subqueries effectively
       act as temporary tables or views for the duration of the primary query.
       Each subquery can be a SELECT, VALUES, INSERT, UPDATE or DELETE
       statement. When writing a data-modifying statement (INSERT, UPDATE or
       DELETE) in WITH, it is usual to include a RETURNING clause. It is the
       output of RETURNING, not the underlying table that the statement
       modifies, that forms the temporary table that is read by the primary
       query. If RETURNING is omitted, the statement is still executed, but it
       produces no output so it cannot be referenced as a table by the primary
       query.

       A name (without schema qualification) must be specified for each WITH
       query. Optionally, a list of column names can be specified; if this is
       omitted, the column names are inferred from the subquery.

       If RECURSIVE is specified, it allows a SELECT subquery to reference
       itself by name. Such a subquery must have the form

	   non_recursive_term UNION [ ALL | DISTINCT ] recursive_term

       where the recursive self-reference must appear on the right-hand side
       of the UNION. Only one recursive self-reference is permitted per query.
       Recursive data-modifying statements are not supported, but you can use
       the results of a recursive SELECT query in a data-modifying statement.
       See Section 7.8, "WITH Queries (Common Table Expressions)", in the
       documentation for an example.

       Another effect of RECURSIVE is that WITH queries need not be ordered: a
       query can reference another one that is later in the list. (However,
       circular references, or mutual recursion, are not implemented.) Without
       RECURSIVE, WITH queries can only reference sibling WITH queries that
       are earlier in the WITH list.

       A key property of WITH queries is that they are evaluated only once per
       execution of the primary query, even if the primary query refers to
       them more than once. In particular, data-modifying statements are
       guaranteed to be executed once and only once, regardless of whether the
       primary query reads all or any of their output.

       The primary query and the WITH queries are all (notionally) executed at
       the same time. This implies that the effects of a data-modifying
       statement in WITH cannot be seen from other parts of the query, other
       than by reading its RETURNING output. If two such data-modifying
       statements attempt to modify the same row, the results are unspecified.

       See Section 7.8, "WITH Queries (Common Table Expressions)", in the
       documentation for additional information.

   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 before the table name, only that table
	   is scanned. If ONLY is not specified, the table and all its
	   descendant tables (if any) are scanned. Optionally, * can be
	   specified after the table name to explicitly indicate that
	   descendant tables are included.

       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. A VALUES(7)
	   command can also be used here.

       with_query_name
	   A WITH query is referenced by writing its name, just as though the
	   query's name were a table name. (In fact, the WITH query hides any
	   real table of the same name for the purposes of the primary query.
	   If necessary, you can refer to a real table of the same name by
	   schema-qualifying the table's name.) An alias can be provided in
	   the same way as for a table.

       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 can 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

	   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 can 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 matching names. If there are no common
	   column names, NATURAL is equivalent to ON TRUE.

   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 or if the ungrouped column is
       functionally dependent on the grouped columns, since there would
       otherwise be more than one possible value to return for an ungrouped
       column. A functional dependency exists if the grouped columns (or a
       subset thereof) are the primary key of the table containing the
       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.

       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.

   WINDOW Clause
       The optional WINDOW clause has the general form

	   WINDOW window_name AS ( window_definition ) [, ...]

       where window_name is a name that can be referenced from OVER clauses or
       subsequent window definitions, and window_definition is

	   [ existing_window_name ]
	   [ PARTITION BY expression [, ...] ]
	   [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
	   [ frame_clause ]

       If an existing_window_name is specified it must refer to an earlier
       entry in the WINDOW list; the new window copies its partitioning clause
       from that entry, as well as its ordering clause if any. In this case
       the new window cannot specify its own PARTITION BY clause, and it can
       specify ORDER BY only if the copied window does not have one. The new
       window always uses its own frame clause; the copied window must not
       specify a frame clause.

       The elements of the PARTITION BY list are interpreted in much the same
       fashion as elements of a GROUP BY Clause, except that they are always
       simple expressions and never the name or number of an output column.
       Another difference is that these expressions can contain aggregate
       function calls, which are not allowed in a regular GROUP BY clause.
       They are allowed here because windowing occurs after grouping and
       aggregation.

       Similarly, the elements of the ORDER BY list are interpreted in much
       the same fashion as elements of an ORDER BY Clause, except that the
       expressions are always taken as simple expressions and never the name
       or number of an output column.

       The optional frame_clause defines the window frame for window functions
       that depend on the frame (not all do). The window frame is a set of
       related rows for each row of the query (called the current row). The
       frame_clause can be one of

	   { RANGE | ROWS } frame_start
	   { RANGE | ROWS } BETWEEN frame_start AND frame_end

       where frame_start and frame_end can be one of

	   UNBOUNDED PRECEDING
	   value PRECEDING
	   CURRENT ROW
	   value FOLLOWING
	   UNBOUNDED FOLLOWING

       If frame_end is omitted it defaults to CURRENT ROW. Restrictions are
       that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be
       UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in
       the above list than the frame_start choice -- for example RANGE BETWEEN
       CURRENT ROW AND value PRECEDING is not allowed.

       The default framing option is RANGE UNBOUNDED PRECEDING, which is the
       same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the
       frame to be all rows from the partition start up through the current
       row's last peer in the ORDER BY ordering (which means all rows if there
       is no ORDER BY). In general, UNBOUNDED PRECEDING means that the frame
       starts with the first row of the partition, and similarly UNBOUNDED
       FOLLOWING means that the frame ends with the last row of the partition
       (regardless of RANGE or ROWS mode). In ROWS mode, CURRENT ROW means
       that the frame starts or ends with the current row; but in RANGE mode
       it means that the frame starts or ends with the current row's first or
       last peer in the ORDER BY ordering. The valuePRECEDING and
       valueFOLLOWING cases are currently only allowed in ROWS mode. They
       indicate that the frame starts or ends with the row that many rows
       before or after the current row.	 value must be an integer expression
       not containing any variables, aggregate functions, or window functions.
       The value must not be null or negative; but it can be zero, which
       selects the current row itself.

       Beware that the ROWS options can produce unpredictable results if the
       ORDER BY ordering does not order the rows uniquely. The RANGE options
       are designed to ensure that rows that are peers in the ORDER BY
       ordering are treated alike; any two peer rows will be both in or both
       not in the frame.

       The purpose of a WINDOW clause is to specify the behavior of window
       functions appearing in the query's SELECT List or ORDER BY Clause.
       These functions can reference the WINDOW clause entries by name in
       their OVER clauses. A WINDOW clause entry does not have to be
       referenced anywhere, however; if it is not used in the query it is
       simply ignored. It is possible to use window functions without any
       WINDOW clause at all, since a window function call can specify its
       window definition directly in its OVER clause. However, the WINDOW
       clause saves typing when the same window definition is needed for more
       than one window function.

       Window functions are described in detail in Section 3.5, "Window
       Functions", in the documentation, Section 4.2.8, "Window Function
       Calls", in the documentation, and Section 7.2.4, "Window Function
       Processing", in the documentation.

   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.

       Just as in a table, every output column of a SELECT has a name. In a
       simple SELECT this name is just used to label the column for display,
       but when the SELECT is a sub-query of a larger query, the name is seen
       by the larger query as the column name of the virtual table produced by
       the sub-query. To specify the name to use for an output column, write
       ASoutput_name after the column's expression. (You can omit AS, but only
       if the desired output name does not match any PostgreSQL keyword (see
       Appendix C, SQL Key Words). For protection against possible future
       keyword additions, it is recommended that you always either write AS or
       double-quote the output name.) If you do not specify a column name, a
       name is chosen automatically by PostgreSQL. If the column's expression
       is a simple column reference then the chosen name is the same as that
       column's name. In more complex cases a function or type name may be
       used, or the system may fall back on a generated name such as ?column?.

       An output column's name can 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, you can write
       table_name.*  as a shorthand for the columns coming from just that
       table. In these cases it is not possible to specify new names with AS;
       the output column names will be the same as the table columns' names.

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

       SELECT 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.

   UNION Clause
       The UNION clause has this general form:

	   select_statement UNION [ ALL | DISTINCT ] 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 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.)  DISTINCT can be written to explicitly specify the
       default behavior of eliminating duplicate rows.

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

       Currently, FOR UPDATE and FOR SHARE cannot 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 | DISTINCT ] 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
       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.	 DISTINCT can be written to explicitly specify
       the default behavior of eliminating duplicate rows.

       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 cannot 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 | DISTINCT ] 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.  DISTINCT can be written to explicitly specify the
       default behavior of eliminating duplicate rows.

       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 cannot 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 ] [ NULLS { FIRST | LAST } ] [, ...]

       The ORDER BY clause causes the result rows to be sorted according to
       the specified expression(s). If two rows are equal according to the
       leftmost expression, they 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.

       Each 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 ordinal number refers to the ordinal (left-to-right) position of
       the output 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 an output 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 output 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 can only specify an
       output column name or number, not an expression.

       If an ORDER BY expression is a simple name that matches both an output
       column name and an input column name, ORDER BY will interpret it as the
       output 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 can 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 can be specified in the USING clause. An
       ordering operator must be a less-than or greater-than member of some
       B-tree operator family.	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.)

       If NULLS LAST is specified, null values sort after all non-null values;
       if NULLS FIRST is specified, null values sort before all non-null
       values. If neither is specified, the default behavior is NULLS LAST
       when ASC is specified or implied, and NULLS FIRST when DESC is
       specified (thus, the default is to act as though nulls are larger than
       non-nulls). When USING is specified, the default nulls ordering depends
       on whether the operator is a less-than or greater-than operator.

       Note that ordering options apply only to the expression they follow;
       for example ORDER BY x, y DESC does not mean the same thing as ORDER BY
       x DESC, y DESC.

       Character-string data is sorted according to the collation that applies
       to the column being sorted. That can be overridden at need by including
       a COLLATE clause in the expression, for example ORDER BY mycolumn
       COLLATE "en_US". For more information see Section 4.2.10, "Collation
       Expressions", in the documentation and Section 22.2, "Collation
       Support", in the documentation.

   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.

       If the count expression evaluates to NULL, it is treated as LIMIT ALL,
       i.e., no limit. If start evaluates to NULL, it is treated the same as
       OFFSET 0.

       SQL:2008 introduced a different syntax to achieve the same result,
       which PostgreSQL also supports. It is:

	   OFFSET start { ROW | ROWS }
	   FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY

       In this syntax, to write anything except a simple integer constant for
       start or count, you must write parentheses around it. If count is
       omitted in a FETCH clause, it defaults to 1.  ROW and ROWS as well as
       FIRST and NEXT are noise words that don't influence the effects of
       these clauses. According to the standard, the OFFSET clause must come
       before the FETCH clause if both are present; but PostgreSQL is laxer
       and allows either order.

       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 might 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.

       It is even possible for repeated executions of the same LIMIT query to
       return different subsets of the rows of a table, if there is not an
       ORDER BY to enforce selection of a deterministic subset. Again, this is
       not a bug; determinism of the results is simply not guaranteed in such
       a case.

   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).
       Within a REPEATABLE READ or SERIALIZABLE transaction, however, an error
       will be thrown if a row to be locked has changed since the transaction
       started. For further discussion see Chapter 13, Concurrency Control, in
       the documentation.

       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.

       To prevent the operation from waiting for other transactions to commit,
       use the NOWAIT option. With NOWAIT, the statement 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
       Chapter 13, Concurrency Control, in the documentation). You can use
       LOCK(7) with the NOWAIT option first, if you need to acquire the
       table-level lock without waiting.

       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
       without a table list affects all tables used in the statement. 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. However, FOR UPDATE/FOR SHARE do
       not apply to WITH queries referenced by the primary query. If you want
       row locking to occur within a WITH query, specify FOR UPDATE or FOR
       SHARE within the WITH query.

       Multiple FOR UPDATE and FOR SHARE clauses can be written if it is
       necessary 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
       cannot be clearly identified with individual table rows; for example
       they cannot be used with aggregation.

       When FOR UPDATE or FOR SHARE appears at the top level of a SELECT
       query, the rows that are locked are exactly those that are returned by
       the query; in the case of a join query, the rows locked are those that
       contribute to returned join rows. In addition, rows that satisfied the
       query conditions as of the query snapshot will be locked, although they
       will not be returned if they were updated after the snapshot and no
       longer satisfy the query conditions. If a LIMIT is used, locking stops
       once enough rows have been returned to satisfy the limit (but note that
       rows skipped over by OFFSET will get locked). Similarly, if FOR UPDATE
       or FOR SHARE is used in a cursor's query, only rows actually fetched or
       stepped past by the cursor will be locked.

       When FOR UPDATE or FOR SHARE appears in a sub-SELECT, the rows locked
       are those returned to the outer query by the sub-query. This might
       involve fewer rows than inspection of the sub-query alone would
       suggest, since conditions from the outer query might be used to
       optimize execution of the sub-query. For example,

	   SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss WHERE col1 = 5;

       will lock only rows having col1 = 5, even though that condition is not
       textually within the sub-query.

	   Caution
	   Avoid locking a row and then modifying it within a later savepoint
	   or PL/pgSQL exception block. A subsequent rollback 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
	   modified. This hazard occurs if a row locked in the current
	   transaction is updated or deleted, or if a shared lock is upgraded
	   to exclusive: in all these cases, the former lock state is
	   forgotten. 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
	   PostgreSQL.

	   Caution
	   It is possible for a SELECT command running at the READ COMMITTED
	   transaction isolation level and using ORDER BY and FOR UPDATE/SHARE
	   to return rows out of order. This is because ORDER BY is applied
	   first. The command sorts the result, but might then block trying to
	   obtain a lock on one or more of the rows. Once the SELECT unblocks,
	   some of the ordering column values might have been modified,
	   leading to those rows appearing to be out of order (though they are
	   in order in terms of the original column values). This can be
	   worked around at need by placing the FOR UPDATE/SHARE clause in a
	   sub-query, for example

	       SELECT * FROM (SELECT * FROM mytable FOR UPDATE) ss ORDER BY column1;

	   Note that this will result in locking all rows of mytable, whereas
	   FOR UPDATE at the top level would lock only the actually returned
	   rows. This can make for a significant performance difference,
	   particularly if the ORDER BY is combined with LIMIT or other
	   restrictions. So this technique is recommended only if concurrent
	   updates of the ordering columns are expected and a strictly sorted
	   result is required.

	   At the REPEATABLE READ or SERIALIZABLE transaction isolation level
	   this would cause a serialization failure (with a SQLSTATE of
	   '40001'), so there is no possibility of receiving rows out of order
	   under these isolation levels.

   TABLE Command
       The command

	   TABLE name

       is completely equivalent to

	   SELECT * FROM name

       It can be used as a top-level command or as a space-saving syntax
       variant in parts of complex queries.

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

       This example shows how to use a simple WITH clause:

	   WITH t AS (
	       SELECT random() as x FROM generate_series(1, 3)
	     )
	   SELECT * FROM t
	   UNION ALL
	   SELECT * FROM t

		    x
	   --------------------
	     0.534150459803641
	     0.520092216785997
	    0.0735620250925422
	     0.534150459803641
	     0.520092216785997
	    0.0735620250925422

       Notice that the WITH query was evaluated only once, so that we got two
       sets of the same three random values.

       This example uses WITH RECURSIVE to find all subordinates (direct or
       indirect) of the employee Mary, and their level of indirectness, from a
       table that shows only direct subordinates:

	   WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
	       SELECT 1, employee_name, manager_name
	       FROM employee
	       WHERE manager_name = 'Mary'
	     UNION ALL
	       SELECT er.distance + 1, e.employee_name, e.manager_name
	       FROM employee_recursive er, employee e
	       WHERE er.employee_name = e.manager_name
	     )
	   SELECT distance, employee_name FROM employee_recursive;

       Notice the typical form of recursive queries: an initial condition,
       followed by UNION, followed by the recursive part of the query. Be sure
       that the recursive part of the query will eventually return no tuples,
       or else the query will loop indefinitely. (See Section 7.8, "WITH
       Queries (Common Table Expressions)", in the documentation for more
       examples.)

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
       referenced by the query. This is no longer allowed.

   Omitting the AS Key Word
       In the SQL standard, the optional key word AS can be omitted before an
       output column name whenever the new column name is a valid column name
       (that is, not the same as any reserved keyword).	 PostgreSQL is
       slightly more restrictive: AS is required if the new column name
       matches any keyword at all, reserved or not. Recommended practice is to
       use AS or double-quote output column names, to prevent any possible
       conflict against future keyword additions.

       In FROM items, both the standard and PostgreSQL allow AS to be omitted
       before an alias that is an unreserved keyword. But this is impractical
       for output column names, because of syntactic ambiguities.

   ONLY and Inheritance
       The SQL standard requires parentheses around the table name when
       writing ONLY, for example SELECT * FROM ONLY (tab1), ONLY (tab2) WHERE
       ....  PostgreSQL considers these parentheses to be optional.

       PostgreSQL allows a trailing * to be written to explicitly specify the
       non-ONLY behavior of including child tables. The standard does not
       allow this.

       (These points apply equally to all SQL commands supporting the ONLY
       option.)

   Namespace Available to GROUP BY and ORDER BY
       In the SQL-92 standard, an ORDER BY clause can only use output column
       names or numbers, while a GROUP BY clause can 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
       output-column names.

       SQL:1999 and later use 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.

   Functional Dependencies
       PostgreSQL recognizes functional dependency (allowing columns to be
       omitted from GROUP BY) only when a table's primary key is included in
       the GROUP BY list. The SQL standard specifies additional conditions
       that should be recognized.

   WINDOW Clause Restrictions
       The SQL standard provides additional options for the window
       frame_clause.  PostgreSQL currently supports only the options listed
       above.

   LIMIT and OFFSET
       The clauses LIMIT and OFFSET are PostgreSQL-specific syntax, also used
       by MySQL. The SQL:2008 standard has introduced the clauses OFFSET ...
       FETCH {FIRST|NEXT} ...  for the same functionality, as shown above in
       LIMIT Clause. This syntax is also used by IBM DB2. (Applications
       written for Oracle frequently use a workaround involving the
       automatically generated rownum column, which is not available in
       PostgreSQL, to implement the effects of these clauses.)

   FOR UPDATE and FOR SHARE
       Although FOR UPDATE appears in the SQL standard, the standard allows it
       only as an option of DECLARE CURSOR.  PostgreSQL allows it in any
       SELECT query as well as in sub-SELECTs, but this is an extension. The
       FOR SHARE variant, and the NOWAIT option, do not appear in the
       standard.

   Data-Modifying Statements in WITH
       PostgreSQL allows INSERT, UPDATE, and DELETE to be used as WITH
       queries. This is not found in the SQL standard.

   Nonstandard Clauses
       The clause DISTINCT ON is not defined in the SQL standard.



PostgreSQL 9.2.24		  2017-11-06			     SELECT(7)