select

SELECT(7)                        SQL Commands                        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 ] 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 )

       TABLE { [ ONLY ] table_name [ * ] | with_query_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 [select(7)] 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 [select(7)] below.)

       3.     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.)

       4.     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.)

       5.     The actual output rows are computed using the SELECT output
              expressions for each selected row. (See SELECT List [select(7)]
              below.)

       6.     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(7)], and EXCEPT Clause [select(7)] below.)

       7.     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.)

       8.     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.)

       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 [select(7)] 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 [select(7)] 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.

       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 subquery to reference itself by
       name. Such a subquery must have the form

       non_recursive_term UNION [ ALL ] 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.

       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 useful 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.

       See 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 [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

              · [ 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 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 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.

       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 [select(7)], 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 [select(7)], 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). It can be one of

       RANGE UNBOUNDED PRECEDING
       RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
       RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
       ROWS UNBOUNDED PRECEDING
       ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
       ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING

       The first two are equivalent and are also the default: they set 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). The options RANGE BETWEEN UNBOUNDED PRECEDING AND
       UNBOUNDED FOLLOWING and ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED
       FOLLOWING are also equivalent: they always select all rows in the
       partition.  Lastly, ROWS UNBOUNDED PRECEDING or its verbose equivalent
       ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW select all rows up
       through the current row (regardless of duplicates).  Beware that this
       option can produce implementation-dependent results if the ORDER BY
       ordering does not order the rows uniquely.

       The purpose of a WINDOW clause is to specify the behavior of window
       functions appearing in the query's SELECT List [select(7)] or ORDER BY
       Clause [select(7)]. 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 in the documentation, in
       the documentation, and 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
       AS output_name after the column's expression. (You can omit AS, but
       only if the desired output name does not match any PostgreSQL keyword
       (see in the documentation). 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 generated name looking like
       ?columnN? is usually chosen.

       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.

   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
       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 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 ] 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.

       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 ] 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 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 locale-specific
       collation order that was established when the database was created.

   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.

       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 thing, which
       PostgreSQL also supports. It is:

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

       Both clauses are optional, but if present the OFFSET clause must come
       before the FETCH clause. ROW and ROWS as well as FIRST and NEXT are
       noise words that don't influence the effects of these clauses. In this
       syntax, when using expressions other than simple constants for start or
       count, parentheses will be necessary in most cases. If count is omitted
       in FETCH, it defaults to 1.

       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).
       For further discussion see in the documentation.

       To prevent the operation from waiting for other transactions to commit,
       use the NOWAIT option. SELECT FOR UPDATE NOWAIT reports an error,
       rather than waiting, if a selected row cannot be locked immediately.
       Note that NOWAIT applies only to the row-level lock(s) — the required
       ROW SHARE table-level lock is still taken in the ordinary way (see in
       the documentation). You can use the NOWAIT option of LOCK [lock(7)] if
       you need to acquire the table-level lock without waiting.

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

       If specific tables are named in FOR UPDATE or FOR SHARE, then only rows
       coming from those tables are locked; any other tables used in the
       SELECT are simply read as usual. A FOR UPDATE or FOR SHARE clause
       without a table list affects all tables used in the command.  If FOR
       UPDATE or FOR SHARE is applied to a view or sub-query, it affects all
       tables used in the view or sub-query.  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.

              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 using both LIMIT
              and FOR UPDATE/SHARE clauses to return fewer rows than specified
              by LIMIT.  This is because LIMIT is applied first. The command
              selects the specified number of rows, but might then block
              trying to obtain a 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.


              Caution: Similarly, it is possible for a SELECT command using
              ORDER BY and FOR UPDATE/SHARE to return rows out of order. This
              is because ORDER BY is applied first. The command orders the
              result, but might then block trying to obtain a lock on one or
              more of the rows. Once the SELECT unblocks, one of the ordered
              columns might have been modified and be returned out of order. A
              workaround is to perform SELECT ... FOR UPDATE/SHARE and then
              SELECT ... ORDER BY.


   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 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 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.

   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.

   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 [select(7)], and this syntax is also used by IBM DB2.
       (Applications written for Oracle frequently use a workaround involving
       the automatically generated rownum column, not available in PostgreSQL,
       to implement the effects of these clauses.)

   NONSTANDARD CLAUSES
       The clause DISTINCT ON is not defined in the SQL standard.



SQL - Language Statements         2014-02-17                         SELECT(7)