A table expression computes a table. The
table expression contains a FROM
clause that is
optionally followed by WHERE
, GROUP BY
, and
HAVING
clauses. Trivial table expressions simply refer
to a table on disk, a so-called base table, but more complex
expressions can be used to modify or combine base tables in various
ways.
The optional WHERE
, GROUP BY
, and
HAVING
clauses in the table expression specify a
pipeline of successive transformations performed on the table
derived in the FROM
clause. All these transformations
produce a virtual table that provides the rows that are passed to
the select list to compute the output rows of the query.
FROM
Clause #
The FROM
clause derives a
table from one or more other tables given in a comma-separated
table reference list.
FROMtable_reference
[,table_reference
[, ...]]
A table reference can be a table name (possibly schema-qualified),
or a derived table such as a subquery, a JOIN
construct, or
complex combinations of these. If more than one table reference is
listed in the FROM
clause, the tables are cross-joined
(that is, the Cartesian product of their rows is formed; see below).
The result of the FROM
list is an intermediate virtual
table that can then be subject to
transformations by the WHERE
, GROUP BY
,
and HAVING
clauses and is finally the result of the
overall table expression.
When a table reference names a table that is the parent of a
table inheritance hierarchy, the table reference produces rows of
not only that table but all of its descendant tables, unless the
key word ONLY
precedes the table name. However, the
reference produces only the columns that appear in the named table
— any columns added in subtables are ignored.
Instead of writing ONLY
before the table name, you can write
*
after the table name to explicitly specify that descendant
tables are included. There is no real reason to use this syntax any more,
because searching descendant tables is now always the default behavior.
However, it is supported for compatibility with older releases.
A joined table is a table derived from two other (real or derived) tables according to the rules of the particular join type. Inner, outer, and cross-joins are available. The general syntax of a joined table is
T1
join_type
T2
[join_condition
]
Joins of all types can be chained together, or nested: either or
both T1
and
T2
can be joined tables. Parentheses
can be used around JOIN
clauses to control the join
order. In the absence of parentheses, JOIN
clauses
nest left-to-right.
Join Types
T1
CROSS JOINT2
For every possible combination of rows from
T1
and
T2
(i.e., a Cartesian product),
the joined table will contain a
row consisting of all columns in T1
followed by all columns in T2
. If
the tables have N and M rows respectively, the joined
table will have N * M rows.
FROM
is equivalent to
T1
CROSS JOIN
T2
FROM
(see below).
It is also equivalent to
T1
INNER JOIN
T2
ON TRUEFROM
.
T1
,
T2
This latter equivalence does not hold exactly when more than two
tables appear, because JOIN
binds more tightly than
comma. For example
FROM
is not the same as
T1
CROSS JOIN
T2
INNER JOIN T3
ON condition
FROM
because the T1
,
T2
INNER JOIN T3
ON condition
condition
can
reference T1
in the first case but not
the second.
T1
{ [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOINT2
ONboolean_expression
T1
{ [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOINT2
USING (join column list
)T1
NATURAL { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOINT2
The words INNER
and
OUTER
are optional in all forms.
INNER
is the default;
LEFT
, RIGHT
, and
FULL
imply an outer join.
The join condition is specified in the
ON
or USING
clause, or implicitly by
the word NATURAL
. The join condition determines
which rows from the two source tables are considered to
“match”, as explained in detail below.
The possible types of qualified join are:
INNER JOIN
For each row R1 of T1, the joined table has a row for each row in T2 that satisfies the join condition with R1.
LEFT OUTER JOIN
First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row is added with null values in columns of T2. Thus, the joined table always has at least one row for each row in T1.
RIGHT OUTER JOIN
First, an inner join is performed. Then, for each row in T2 that does not satisfy the join condition with any row in T1, a joined row is added with null values in columns of T1. This is the converse of a left join: the result table will always have a row for each row in T2.
FULL OUTER JOIN
First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row is added with null values in columns of T2. Also, for each row of T2 that does not satisfy the join condition with any row in T1, a joined row with null values in the columns of T1 is added.
The ON
clause is the most general kind of join
condition: it takes a Boolean value expression of the same
kind as is used in a WHERE
clause. A pair of rows
from T1
and T2
match if the
ON
expression evaluates to true.
The USING
clause is a shorthand that allows you to take
advantage of the specific situation where both sides of the join use
the same name for the joining column(s). It takes a
comma-separated list of the shared column names
and forms a join condition that includes an equality comparison
for each one. For example, joining T1
and T2
with USING (a, b)
produces
the join condition ON
.
T1
.a
= T2
.a AND T1
.b
= T2
.b
Furthermore, the output of JOIN USING
suppresses
redundant columns: there is no need to print both of the matched
columns, since they must have equal values. While JOIN
ON
produces all columns from T1
followed by all
columns from T2
, JOIN USING
produces one
output column for each of the listed column pairs (in the listed
order), followed by any remaining columns from T1
,
followed by any remaining columns from T2
.
Finally, NATURAL
is a shorthand form of
USING
: it forms a USING
list
consisting of all column names that appear in both
input tables. As with USING
, these columns appear
only once in the output table. If there are no common
column names, NATURAL JOIN
behaves like
JOIN ... ON TRUE
, producing a cross-product join.
USING
is reasonably safe from column changes
in the joined relations since only the listed columns
are combined. NATURAL
is considerably more risky since
any schema changes to either relation that cause a new matching
column name to be present will cause the join to combine that new
column as well.
To put this together, assume we have tables t1
:
num | name -----+------ 1 | a 2 | b 3 | c
and t2
:
num | value -----+------- 1 | xxx 3 | yyy 5 | zzz
then we get the following results for the various joins:
=>
SELECT * FROM t1 CROSS JOIN t2;
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 1 | a | 3 | yyy 1 | a | 5 | zzz 2 | b | 1 | xxx 2 | b | 3 | yyy 2 | b | 5 | zzz 3 | c | 1 | xxx 3 | c | 3 | yyy 3 | c | 5 | zzz (9 rows)=>
SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num;
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy (2 rows)=>
SELECT * FROM t1 INNER JOIN t2 USING (num);
num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows)=>
SELECT * FROM t1 NATURAL INNER JOIN t2;
num | name | value -----+------+------- 1 | a | xxx 3 | c | yyy (2 rows)=>
SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num;
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy (3 rows)=>
SELECT * FROM t1 LEFT JOIN t2 USING (num);
num | name | value -----+------+------- 1 | a | xxx 2 | b | 3 | c | yyy (3 rows)=>
SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num;
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 3 | c | 3 | yyy | | 5 | zzz (3 rows)=>
SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num;
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | 3 | yyy | | 5 | zzz (4 rows)
The join condition specified with ON
can also contain
conditions that do not relate directly to the join. This can
prove useful for some queries but needs to be thought out
carefully. For example:
=>
SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx';
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx 2 | b | | 3 | c | | (3 rows)
Notice that placing the restriction in the WHERE
clause
produces a different result:
=>
SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num WHERE t2.value = 'xxx';
num | name | num | value -----+------+-----+------- 1 | a | 1 | xxx (1 row)
This is because a restriction placed in the ON
clause is processed before the join, while
a restriction placed in the WHERE
clause is processed
after the join.
That does not matter with inner joins, but it matters a lot with outer
joins.
A temporary name can be given to tables and complex table references to be used for references to the derived table in the rest of the query. This is called a table alias.
To create a table alias, write
FROMtable_reference
ASalias
or
FROMtable_reference
alias
The AS
key word is optional noise.
alias
can be any identifier.
A typical application of table aliases is to assign short identifiers to long table names to keep the join clauses readable. For example:
SELECT * FROM some_very_long_table_name s JOIN another_fairly_long_name a ON s.id = a.num;
The alias becomes the new name of the table reference so far as the current query is concerned — it is not allowed to refer to the table by the original name elsewhere in the query. Thus, this is not valid:
SELECT * FROM my_table AS m WHERE my_table.a > 5; -- wrong
Table aliases are mainly for notational convenience, but it is necessary to use them when joining a table to itself, e.g.:
SELECT * FROM people AS mother JOIN people AS child ON mother.id = child.mother_id;
Parentheses are used to resolve ambiguities. In the following example,
the first statement assigns the alias b
to the second
instance of my_table
, but the second statement assigns the
alias to the result of the join:
SELECT * FROM my_table AS a CROSS JOIN my_table AS b ... SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...
Another form of table aliasing gives temporary names to the columns of the table, as well as the table itself:
FROMtable_reference
[AS]alias
(column1
[,column2
[, ...]] )
If fewer column aliases are specified than the actual table has columns, the remaining columns are not renamed. This syntax is especially useful for self-joins or subqueries.
When an alias is applied to the output of a JOIN
clause, the alias hides the original
name(s) within the JOIN
. For example:
SELECT a.* FROM my_table AS a JOIN your_table AS b ON ...
is valid SQL, but:
SELECT a.* FROM (my_table AS a JOIN your_table AS b ON ...) AS c
is not valid; the table alias a
is not visible
outside the alias c
.
Subqueries specifying a derived table must be enclosed in parentheses. They may be assigned a table alias name, and optionally column alias names (as in Section 7.2.1.2). For example:
FROM (SELECT * FROM table1) AS alias_name
This example is equivalent to FROM table1 AS
alias_name
. More interesting cases, which cannot be
reduced to a plain join, arise when the subquery involves
grouping or aggregation.
A subquery can also be a VALUES
list:
FROM (VALUES ('anne', 'smith'), ('bob', 'jones'), ('joe', 'blow')) AS names(first, last)
Again, a table alias is optional. Assigning alias names to the columns
of the VALUES
list is optional, but is good practice.
For more information see Section 7.7.
According to the SQL standard, a table alias name must be supplied
for a subquery. PostgreSQL
allows AS
and the alias to be omitted, but
writing one is good practice in SQL code that might be ported to
another system.
Table functions are functions that produce a set of rows, made up
of either base data types (scalar types) or composite data types
(table rows). They are used like a table, view, or subquery in
the FROM
clause of a query. Columns returned by table
functions can be included in SELECT
,
JOIN
, or WHERE
clauses in the same manner
as columns of a table, view, or subquery.
Table functions may also be combined using the ROWS FROM
syntax, with the results returned in parallel columns; the number of
result rows in this case is that of the largest function result, with
smaller results padded with null values to match.
function_call
[WITH ORDINALITY] [[AS]table_alias
[(column_alias
[, ... ])]] ROWS FROM(function_call
[, ... ] ) [WITH ORDINALITY] [[AS]table_alias
[(column_alias
[, ... ])]]
If the WITH ORDINALITY
clause is specified, an
additional column of type bigint
will be added to the
function result columns. This column numbers the rows of the function
result set, starting from 1. (This is a generalization of the
SQL-standard syntax for UNNEST ... WITH ORDINALITY
.)
By default, the ordinal column is called ordinality
, but
a different column name can be assigned to it using
an AS
clause.
The special table function UNNEST
may be called with
any number of array parameters, and it returns a corresponding number of
columns, as if UNNEST
(Section 9.19) had been called on each parameter
separately and combined using the ROWS FROM
construct.
UNNEST(array_expression
[, ... ] ) [WITH ORDINALITY] [[AS]table_alias
[(column_alias
[, ... ])]]
If no table_alias
is specified, the function
name is used as the table name; in the case of a ROWS FROM()
construct, the first function's name is used.
If column aliases are not supplied, then for a function returning a base data type, the column name is also the same as the function name. For a function returning a composite type, the result columns get the names of the individual attributes of the type.
Some examples:
CREATE TABLE foo (fooid int, foosubid int, fooname text); CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$ SELECT * FROM foo WHERE fooid = $1; $$ LANGUAGE SQL; SELECT * FROM getfoo(1) AS t1; SELECT * FROM foo WHERE foosubid IN ( SELECT foosubid FROM getfoo(foo.fooid) z WHERE z.fooid = foo.fooid ); CREATE VIEW vw_getfoo AS SELECT * FROM getfoo(1); SELECT * FROM vw_getfoo;
In some cases it is useful to define table functions that can
return different column sets depending on how they are invoked.
To support this, the table function can be declared as returning
the pseudo-type record
with no OUT
parameters. When such a function is used in
a query, the expected row structure must be specified in the
query itself, so that the system can know how to parse and plan
the query. This syntax looks like:
function_call
[AS]alias
(column_definition
[, ... ])function_call
AS [alias
] (column_definition
[, ... ]) ROWS FROM( ...function_call
AS (column_definition
[, ... ]) [, ... ] )
When not using the ROWS FROM()
syntax,
the column_definition
list replaces the column
alias list that could otherwise be attached to the FROM
item; the names in the column definitions serve as column aliases.
When using the ROWS FROM()
syntax,
a column_definition
list can be attached to
each member function separately; or if there is only one member function
and no WITH ORDINALITY
clause,
a column_definition
list can be written in
place of a column alias list following ROWS FROM()
.
Consider this example:
SELECT * FROM dblink('dbname=mydb', 'SELECT proname, prosrc FROM pg_proc') AS t1(proname name, prosrc text) WHERE proname LIKE 'bytea%';
The dblink function
(part of the dblink module) executes
a remote query. It is declared to return
record
since it might be used for any kind of query.
The actual column set must be specified in the calling query so
that the parser knows, for example, what *
should
expand to.
This example uses ROWS FROM
:
SELECT * FROM ROWS FROM ( json_to_recordset('[{"a":40,"b":"foo"},{"a":"100","b":"bar"}]') AS (a INTEGER, b TEXT), generate_series(1, 3) ) AS x (p, q, s) ORDER BY p; p | q | s -----+-----+--- 40 | foo | 1 100 | bar | 2 | | 3
It joins two functions into a single FROM
target. json_to_recordset()
is instructed
to return two columns, the first integer
and the second text
. The result of
generate_series()
is used directly.
The ORDER BY
clause sorts the column values
as integers.
LATERAL
Subqueries #
Subqueries appearing in FROM
can be
preceded by the key word LATERAL
. This allows them to
reference columns provided by preceding FROM
items.
(Without LATERAL
, each subquery is
evaluated independently and so cannot cross-reference any other
FROM
item.)
Table functions appearing in FROM
can also be
preceded by the key word LATERAL
, but for functions the
key word is optional; the function's arguments can contain references
to columns provided by preceding FROM
items in any case.
A LATERAL
item can appear at top level in the
FROM
list, or within a JOIN
tree. In the latter
case it can also refer to any items that are on the left-hand side of a
JOIN
that it is on the right-hand side of.
When a FROM
item contains LATERAL
cross-references, evaluation proceeds as follows: for each row of the
FROM
item providing the cross-referenced column(s), or
set of rows of multiple FROM
items providing the
columns, the LATERAL
item is evaluated using that
row or row set's values of the columns. The resulting row(s) are
joined as usual with the rows they were computed from. This is
repeated for each row or set of rows from the column source table(s).
A trivial example of LATERAL
is
SELECT * FROM foo, LATERAL (SELECT * FROM bar WHERE bar.id = foo.bar_id) ss;
This is not especially useful since it has exactly the same result as the more conventional
SELECT * FROM foo, bar WHERE bar.id = foo.bar_id;
LATERAL
is primarily useful when the cross-referenced
column is necessary for computing the row(s) to be joined. A common
application is providing an argument value for a set-returning function.
For example, supposing that vertices(polygon)
returns the
set of vertices of a polygon, we could identify close-together vertices
of polygons stored in a table with:
SELECT p1.id, p2.id, v1, v2 FROM polygons p1, polygons p2, LATERAL vertices(p1.poly) v1, LATERAL vertices(p2.poly) v2 WHERE (v1 <-> v2) < 10 AND p1.id != p2.id;
This query could also be written
SELECT p1.id, p2.id, v1, v2 FROM polygons p1 CROSS JOIN LATERAL vertices(p1.poly) v1, polygons p2 CROSS JOIN LATERAL vertices(p2.poly) v2 WHERE (v1 <-> v2) < 10 AND p1.id != p2.id;
or in several other equivalent formulations. (As already mentioned,
the LATERAL
key word is unnecessary in this example, but
we use it for clarity.)
It is often particularly handy to LEFT JOIN
to a
LATERAL
subquery, so that source rows will appear in
the result even if the LATERAL
subquery produces no
rows for them. For example, if get_product_names()
returns
the names of products made by a manufacturer, but some manufacturers in
our table currently produce no products, we could find out which ones
those are like this:
SELECT m.name FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true WHERE pname IS NULL;
WHERE
Clause #
The syntax of the WHERE
clause is
WHERE search_condition
where search_condition
is any value
expression (see Section 4.2) that
returns a value of type boolean
.
After the processing of the FROM
clause is done, each
row of the derived virtual table is checked against the search
condition. If the result of the condition is true, the row is
kept in the output table, otherwise (i.e., if the result is
false or null) it is discarded. The search condition typically
references at least one column of the table generated in the
FROM
clause; this is not required, but otherwise the
WHERE
clause will be fairly useless.
The join condition of an inner join can be written either in
the WHERE
clause or in the JOIN
clause.
For example, these table expressions are equivalent:
FROM a, b WHERE a.id = b.id AND b.val > 5
and:
FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val > 5
or perhaps even:
FROM a NATURAL JOIN b WHERE b.val > 5
Which one of these you use is mainly a matter of style. The
JOIN
syntax in the FROM
clause is
probably not as portable to other SQL database management systems,
even though it is in the SQL standard. For
outer joins there is no choice: they must be done in
the FROM
clause. The ON
or USING
clause of an outer join is not equivalent to a
WHERE
condition, because it results in the addition
of rows (for unmatched input rows) as well as the removal of rows
in the final result.
Here are some examples of WHERE
clauses:
SELECT ... FROM fdt WHERE c1 > 5 SELECT ... FROM fdt WHERE c1 IN (1, 2, 3) SELECT ... FROM fdt WHERE c1 IN (SELECT c1 FROM t2) SELECT ... FROM fdt WHERE c1 IN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10) SELECT ... FROM fdt WHERE c1 BETWEEN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10) AND 100 SELECT ... FROM fdt WHERE EXISTS (SELECT c1 FROM t2 WHERE c2 > fdt.c1)
fdt
is the table derived in the
FROM
clause. Rows that do not meet the search
condition of the WHERE
clause are eliminated from
fdt
. Notice the use of scalar subqueries as
value expressions. Just like any other query, the subqueries can
employ complex table expressions. Notice also how
fdt
is referenced in the subqueries.
Qualifying c1
as fdt.c1
is only necessary
if c1
is also the name of a column in the derived
input table of the subquery. But qualifying the column name adds
clarity even when it is not needed. This example shows how the column
naming scope of an outer query extends into its inner queries.
GROUP BY
and HAVING
Clauses #
After passing the WHERE
filter, the derived input
table might be subject to grouping, using the GROUP BY
clause, and elimination of group rows using the HAVING
clause.
SELECTselect_list
FROM ... [WHERE ...] GROUP BYgrouping_column_reference
[,grouping_column_reference
]...
The GROUP BY
clause is
used to group together those rows in a table that have the same
values in all the columns listed. The order in which the columns
are listed does not matter. The effect is to combine each set
of rows having common values into one group row that
represents all rows in the group. This is done to
eliminate redundancy in the output and/or compute aggregates that
apply to these groups. For instance:
=>
SELECT * FROM test1;
x | y ---+--- a | 3 c | 2 b | 5 a | 1 (4 rows)=>
SELECT x FROM test1 GROUP BY x;
x --- a b c (3 rows)
In the second query, we could not have written SELECT *
FROM test1 GROUP BY x
, because there is no single value
for the column y
that could be associated with each
group. The grouped-by columns can be referenced in the select list since
they have a single value in each group.
In general, if a table is grouped, columns that are not
listed in GROUP BY
cannot be referenced except in aggregate
expressions. An example with aggregate expressions is:
=>
SELECT x, sum(y) FROM test1 GROUP BY x;
x | sum ---+----- a | 4 b | 5 c | 2 (3 rows)
Here sum
is an aggregate function that
computes a single value over the entire group. More information
about the available aggregate functions can be found in Section 9.21.
Grouping without aggregate expressions effectively calculates the
set of distinct values in a column. This can also be achieved
using the DISTINCT
clause (see Section 7.3.3).
Here is another example: it calculates the total sales for each product (rather than the total sales of all products):
SELECT product_id, p.name, (sum(s.units) * p.price) AS sales FROM products p LEFT JOIN sales s USING (product_id) GROUP BY product_id, p.name, p.price;
In this example, the columns product_id
,
p.name
, and p.price
must be
in the GROUP BY
clause since they are referenced in
the query select list (but see below). The column
s.units
does not have to be in the GROUP
BY
list since it is only used in an aggregate expression
(sum(...)
), which represents the sales
of a product. For each product, the query returns a summary row about
all sales of the product.
If the products table is set up so that, say,
product_id
is the primary key, then it would be
enough to group by product_id
in the above example,
since name and price would be functionally
dependent on the product ID, and so there would be no
ambiguity about which name and price value to return for each product
ID group.
In strict SQL, GROUP BY
can only group by columns of
the source table but PostgreSQL extends
this to also allow GROUP BY
to group by columns in the
select list. Grouping by value expressions instead of simple
column names is also allowed.
If a table has been grouped using GROUP BY
,
but only certain groups are of interest, the
HAVING
clause can be used, much like a
WHERE
clause, to eliminate groups from the result.
The syntax is:
SELECTselect_list
FROM ... [WHERE ...] GROUP BY ... HAVINGboolean_expression
Expressions in the HAVING
clause can refer both to
grouped expressions and to ungrouped expressions (which necessarily
involve an aggregate function).
Example:
=>
SELECT x, sum(y) FROM test1 GROUP BY x HAVING sum(y) > 3;
x | sum ---+----- a | 4 b | 5 (2 rows)=>
SELECT x, sum(y) FROM test1 GROUP BY x HAVING x < 'c';
x | sum ---+----- a | 4 b | 5 (2 rows)
Again, a more realistic example:
SELECT product_id, p.name, (sum(s.units) * (p.price - p.cost)) AS profit FROM products p LEFT JOIN sales s USING (product_id) WHERE s.date > CURRENT_DATE - INTERVAL '4 weeks' GROUP BY product_id, p.name, p.price, p.cost HAVING sum(p.price * s.units) > 5000;
In the example above, the WHERE
clause is selecting
rows by a column that is not grouped (the expression is only true for
sales during the last four weeks), while the HAVING
clause restricts the output to groups with total gross sales over
5000. Note that the aggregate expressions do not necessarily need
to be the same in all parts of the query.
If a query contains aggregate function calls, but no GROUP BY
clause, grouping still occurs: the result is a single group row (or
perhaps no rows at all, if the single row is then eliminated by
HAVING
).
The same is true if it contains a HAVING
clause, even
without any aggregate function calls or GROUP BY
clause.
GROUPING SETS
, CUBE
, and ROLLUP
#
More complex grouping operations than those described above are possible
using the concept of grouping sets. The data selected by
the FROM
and WHERE
clauses is grouped separately
by each specified grouping set, aggregates computed for each group just as
for simple GROUP BY
clauses, and then the results returned.
For example:
=>
SELECT * FROM items_sold;
brand | size | sales -------+------+------- Foo | L | 10 Foo | M | 20 Bar | M | 15 Bar | L | 5 (4 rows)=>
SELECT brand, size, sum(sales) FROM items_sold GROUP BY GROUPING SETS ((brand), (size), ());
brand | size | sum -------+------+----- Foo | | 30 Bar | | 20 | L | 15 | M | 35 | | 50 (5 rows)
Each sublist of GROUPING SETS
may specify zero or more columns
or expressions and is interpreted the same way as though it were directly
in the GROUP BY
clause. An empty grouping set means that all
rows are aggregated down to a single group (which is output even if no
input rows were present), as described above for the case of aggregate
functions with no GROUP BY
clause.
References to the grouping columns or expressions are replaced by null values in result rows for grouping sets in which those columns do not appear. To distinguish which grouping a particular output row resulted from, see Table 9.62.
A shorthand notation is provided for specifying two common types of grouping set. A clause of the form
ROLLUP (e1
,e2
,e3
, ... )
represents the given list of expressions and all prefixes of the list including the empty list; thus it is equivalent to
GROUPING SETS ( (e1
,e2
,e3
, ... ), ... (e1
,e2
), (e1
), ( ) )
This is commonly used for analysis over hierarchical data; e.g., total salary by department, division, and company-wide total.
A clause of the form
CUBE (e1
,e2
, ... )
represents the given list and all of its possible subsets (i.e., the power set). Thus
CUBE ( a, b, c )
is equivalent to
GROUPING SETS ( ( a, b, c ), ( a, b ), ( a, c ), ( a ), ( b, c ), ( b ), ( c ), ( ) )
The individual elements of a CUBE
or ROLLUP
clause may be either individual expressions, or sublists of elements in
parentheses. In the latter case, the sublists are treated as single
units for the purposes of generating the individual grouping sets.
For example:
CUBE ( (a, b), (c, d) )
is equivalent to
GROUPING SETS ( ( a, b, c, d ), ( a, b ), ( c, d ), ( ) )
and
ROLLUP ( a, (b, c), d )
is equivalent to
GROUPING SETS ( ( a, b, c, d ), ( a, b, c ), ( a ), ( ) )
The CUBE
and ROLLUP
constructs can be used either
directly in the GROUP BY
clause, or nested inside a
GROUPING SETS
clause. If one GROUPING SETS
clause
is nested inside another, the effect is the same as if all the elements of
the inner clause had been written directly in the outer clause.
If multiple grouping items are specified in a single GROUP BY
clause, then the final list of grouping sets is the cross product of the
individual items. For example:
GROUP BY a, CUBE (b, c), GROUPING SETS ((d), (e))
is equivalent to
GROUP BY GROUPING SETS ( (a, b, c, d), (a, b, c, e), (a, b, d), (a, b, e), (a, c, d), (a, c, e), (a, d), (a, e) )
When specifying multiple grouping items together, the final set of grouping sets might contain duplicates. For example:
GROUP BY ROLLUP (a, b), ROLLUP (a, c)
is equivalent to
GROUP BY GROUPING SETS ( (a, b, c), (a, b), (a, b), (a, c), (a), (a), (a, c), (a), () )
If these duplicates are undesirable, they can be removed using the
DISTINCT
clause directly on the GROUP BY
.
Therefore:
GROUP BY DISTINCT ROLLUP (a, b), ROLLUP (a, c)
is equivalent to
GROUP BY GROUPING SETS ( (a, b, c), (a, b), (a, c), (a), () )
This is not the same as using SELECT DISTINCT
because the output
rows may still contain duplicates. If any of the ungrouped columns contains NULL,
it will be indistinguishable from the NULL used when that same column is grouped.
The construct (a, b)
is normally recognized in expressions as
a row constructor.
Within the GROUP BY
clause, this does not apply at the top
levels of expressions, and (a, b)
is parsed as a list of
expressions as described above. If for some reason you need
a row constructor in a grouping expression, use ROW(a, b)
.
If the query contains any window functions (see
Section 3.5,
Section 9.22 and
Section 4.2.8), these functions are evaluated
after any grouping, aggregation, and HAVING
filtering is
performed. That is, if the query uses any aggregates, GROUP
BY
, or HAVING
, then the rows seen by the window functions
are the group rows instead of the original table rows from
FROM
/WHERE
.
When multiple window functions are used, all the window functions having
syntactically equivalent PARTITION BY
and ORDER BY
clauses in their window definitions are guaranteed to be evaluated in a
single pass over the data. Therefore they will see the same sort ordering,
even if the ORDER BY
does not uniquely determine an ordering.
However, no guarantees are made about the evaluation of functions having
different PARTITION BY
or ORDER BY
specifications.
(In such cases a sort step is typically required between the passes of
window function evaluations, and the sort is not guaranteed to preserve
ordering of rows that its ORDER BY
sees as equivalent.)
Currently, window functions always require presorted data, and so the
query output will be ordered according to one or another of the window
functions' PARTITION BY
/ORDER BY
clauses.
It is not recommended to rely on this, however. Use an explicit
top-level ORDER BY
clause if you want to be sure the
results are sorted in a particular way.