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SELECT() SQL Commands SELECT()
NAME
SELECT - retrieve rows from a table or view
SYNOPSIS
SELECT [ ALL | DISTINCT [ ON ( expression [, ...] ) ] ]
* | expression [ AS output_name ] [, ...]
[ FROM from_item [, ...] ]
[ WHERE condition ]
[ GROUP BY expression [, ...] ]
[ HAVING condition [, ...] ]
[ { UNION | INTERSECT | EXCEPT } [ ALL ] select ]
[ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
[ LIMIT { count | ALL } ]
[ OFFSET start ]
[ FOR { UPDATE | SHARE } [ OF table_name [, ...] ] [ NOWAIT ] [...] ]
where from_item can be one of:
[ ONLY ] table_name [ * ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]
DESCRIPTION
SELECT retrieves rows from zero or more tables. The general processing of SELECT is as
follows:
1. All elements in the FROM list are computed. (Each element in the FROM list is a
real or virtual table.) If more than one element is specified in the FROM list,
they are cross-joined together. (See FROM Clause [select(7)] below.)
2. If the WHERE clause is specified, all rows that do not satisfy the condition are
eliminated from the output. (See WHERE Clause [select(7)] below.)
3. If the GROUP BY clause is specified, the output is divided into groups of rows that
match on one or more values. If the HAVING clause is present, it eliminates groups
that do not satisfy the given condition. (See GROUP BY Clause [select(7)] and HAV-
ING Clause [select(7)] below.)
4. The actual output rows are computed using the SELECT output expressions for each
selected row. (See SELECT List [select(7)] below.)
5. Using the operators UNION, INTERSECT, and EXCEPT, the output of more than one
SELECT statement can be combined to form a single result set. The UNION operator
returns all rows that are in one or both of the result sets. The INTERSECT operator
returns all rows that are strictly in both result sets. The EXCEPT operator returns
the rows that are in the first result set but not in the second. In all three
cases, duplicate rows are eliminated unless ALL is specified. (See UNION Clause
[select(7)], INTERSECT Clause [select(l)], and EXCEPT Clause [select(7)] below.)
6. If the ORDER BY clause is specified, the returned rows are sorted in the specified
order. If ORDER BY is not given, the rows are returned in whatever order the system
finds fastest to produce. (See ORDER BY Clause [select(7)] below.)
7. DISTINCT eliminates duplicate rows from the result. DISTINCT ON eliminates rows
that match on all the specified expressions. ALL (the default) will return all can-
didate rows, including duplicates. (See DISTINCT Clause [select(7)] below.)
8. If the LIMIT or OFFSET clause is specified, the SELECT statement only returns a
subset of the result rows. (See LIMIT Clause [select(7)] below.)
9. 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 a table to read its values. The use of FOR UPDATE or FOR
SHARE requires UPDATE privilege as well.
PARAMETERS
FROM CLAUSE
The FROM clause specifies one or more source tables for the SELECT. If multiple sources
are specified, the result is the Cartesian product (cross join) of all the sources. But
usually qualification conditions are added to restrict the returned rows to a small subset
of the Cartesian product.
The FROM clause can contain the following elements:
table_name
The name (optionally schema-qualified) of an existing table or view. If ONLY is
specified, only that table is scanned. If ONLY is not specified, the table and all
its descendant tables (if any) are scanned. * can be appended to the table name to
indicate that descendant tables are to be scanned, but in the current version, this
is the default behavior. (In releases before 7.1, ONLY was the default behavior.)
The default behavior can be modified by changing the sql_inheritance configuration
option.
alias A substitute name for the FROM item containing the alias. An alias is used for
brevity or to eliminate ambiguity for self-joins (where the same table is scanned
multiple times). When an alias is provided, it completely hides the actual name of
the table or function; for example given FROM foo AS f, the remainder of the SELECT
must refer to this FROM item as f not foo. If an alias is written, a column alias
list can also be written to provide substitute names for one or more columns of the
table.
select A sub-SELECT can appear in the FROM clause. This acts as though its output were
created as a temporary table for the duration of this single SELECT command. Note
that the sub-SELECT must be surrounded by parentheses, and an alias must be pro-
vided for it. A VALUES [values(7)] command can also be used here.
function_name
Function calls can appear in the FROM clause. (This is especially useful for func-
tions 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 func-
tion'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 noth-
ing 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 fil-
ters group rows created by GROUP BY. Each column referenced in condition must unambigu-
ously reference a grouping column, unless the reference appears within an aggregate func-
tion.
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 func-
tions. Such a query will emit a single row if the HAVING condition is true, zero rows if
it is not true.
SELECT LIST
The SELECT list (between the key words SELECT and FROM) specifies expressions that form
the output rows of the SELECT statement. The expressions can (and usually do) refer to
columns computed in the FROM clause. Using the clause AS output_name, another name can be
specified for an output column. This name is primarily used to label the column for dis-
play. It can also be used to refer to the column's value in ORDER BY and GROUP BY clauses,
but not in the WHERE or HAVING clauses; there you must write out the expression instead.
Instead of an expression, * can be written in the output list as a shorthand for all the
columns of the selected rows. Also, one can write table_name.* as a shorthand for the
columns coming from just that table.
UNION CLAUSE
The UNION clause has this general form:
select_statement UNION [ ALL ] select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT, 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 speci-
fied. ALL prevents elimination of duplicates. (Therefore, UNION ALL is usually signifi-
cantly 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 spec-
ified. 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 speci-
fied. 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 expres-
sion(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 result column.
This feature makes it possible to define an ordering on the basis of a column that does
not have a unique name. This is never absolutely necessary because it is always possible
to assign a name to a result column using the AS clause.
It is also possible to use arbitrary expressions in the ORDER BY clause, including columns
that do not appear in the SELECT result list. Thus the following statement is valid:
SELECT name FROM distributors ORDER BY code;
A limitation of this feature is that an ORDER BY clause applying to the result of a UNION,
INTERSECT, or EXCEPT clause can only specify an output column name or number, not an
expression.
If an ORDER BY expression is a simple name that matches both a result column name and an
input column name, ORDER BY will interpret it as the result column name. This is the
opposite of the choice that GROUP BY will make in the same situation. This inconsistency
is made to be compatible with the SQL standard.
Optionally one can add the key word ASC (ascending) or DESC (descending) after any expres-
sion 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 opera-
tor 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 cluster was initialized.
DISTINCT CLAUSE
If DISTINCT is specified, all duplicate rows are removed from the result set (one row is
kept from each group of duplicates). ALL specifies the opposite: all rows are kept; that
is the default.
DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the
given expressions evaluate to equal. The DISTINCT ON expressions are interpreted using the
same rules as for ORDER BY (see above). Note that the ``first row'' of each set is unpre-
dictable unless ORDER BY is used to ensure that the desired row appears first. For exam-
ple:
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 prece-
dence of rows within each DISTINCT ON group.
LIMIT CLAUSE
The LIMIT clause consists of two independent sub-clauses:
LIMIT { count | ALL }
OFFSET start
count specifies the maximum number of rows to return, while start specifies the number of
rows to skip before starting to return rows. When both are specified, start rows are
skipped before starting to count the count rows to be returned.
When using LIMIT, it is a good idea to use an ORDER BY clause that constrains the result
rows into a unique order. Otherwise you will get an unpredictable subset of the query's
rows -- you might be asking for the tenth through twentieth rows, but tenth through twen-
tieth 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 sub-
sets 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.
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 Post-
greSQL.
Caution: It is possible for a SELECT command using both LIMIT and FOR UPDATE/SHARE
clauses to return fewer rows than specified by LIMIT. This is because LIMIT is
applied first. The command selects the specified number of rows, but might then
block trying to obtain lock on one or more of them. Once the SELECT unblocks, the
row might have been deleted or updated so that it does not meet the query WHERE
condition anymore, in which case it will not be returned.
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.
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 col-
umn definition list:
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors(111);
did | name
-----+-------------
111 | Walt Disney
CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
f1 | f2
-----+-------------
111 | Walt Disney
COMPATIBILITY
Of course, the SELECT statement is compatible with the SQL standard. But there are some
extensions and some missing features.
OMITTED FROM CLAUSES
PostgreSQL allows one to omit the FROM clause. It has a straightforward use to compute the
results of simple expressions:
SELECT 2+2;
?column?
----------
4
Some other SQL databases cannot do this except by introducing a dummy one-row table from
which to do the SELECT.
Note that if a FROM clause is not specified, the query cannot reference any database
tables. For example, the following query is invalid:
SELECT distributors.* WHERE distributors.name = 'Westward';
PostgreSQL releases prior to 8.1 would accept queries of this form, and add an implicit
entry to the query's FROM clause for each table 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.
THE AS KEY WORD
In the SQL standard, the optional key word AS is just noise and can be omitted without
affecting the meaning. The PostgreSQL parser requires this key word when renaming output
columns because the type extensibility features lead to parsing ambiguities without it.
AS is optional in FROM items, however.
NAMESPACE AVAILABLE TO GROUP BY AND ORDER BY
In the SQL-92 standard, an ORDER BY clause can only use result 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 stan-
dard's interpretation if there is ambiguity). PostgreSQL also allows both clauses to
specify arbitrary expressions. Note that names appearing in an expression will always be
taken as input-column names, not as result-column names.
SQL:1999 and later use a slightly different definition which is not entirely upward com-
patible with SQL-92. In most cases, however, PostgreSQL will interpret an ORDER BY or
GROUP BY expression the same way SQL:1999 does.
NONSTANDARD CLAUSES
The clauses DISTINCT ON, LIMIT, and OFFSET are not defined in the SQL standard.
SQL - Language Statements 2011-09-22 SELECT()
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