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# GROUP BY

## Description

GROUP BY `GROUPING SETS``CUBE``ROLLUP` is an extension of the GROUP BY clause. It can realize the aggregation of groups of multiple sets in a GROUP BY clause. The result is equivalent to the UNION operation of multiple corresponding GROUP BY clauses.

GROUP BY clause is a special case of GROUP BY GROUPING SETS containing only one element. For example, the GROUPING SETS statement:

``  SELECT a, b, SUM( c ) FROM tab1 GROUP BY GROUPING SETS ( (a, b), (a), (b), ( ) );``

The query result is equivalent to:

``````  SELECT a, b, SUM( c ) FROM tab1 GROUP BY a, b
UNION
SELECT a, null, SUM( c ) FROM tab1 GROUP BY a
UNION
SELECT null, b, SUM( c ) FROM tab1 GROUP BY b
UNION
SELECT null, null, SUM( c ) FROM tab1``````

`GROUPING(expr)` indicates whether a column is an aggregate column. If it is an aggregate column, it is 0, otherwise it is 1.

`GROUPING_ID(expr [ , expr [ , ... ] ])` is similar to GROUPING. GROUPING_ ID calculates the bitmap value of a column list according to the specified column order, and each bit is the value of GROUPING.

GROUPING_ID() function returns the decimal value of the bit vector.

### Syntax

``````  SELECT ...
FROM ...
[ ... ]
GROUP BY [
, ... |
GROUPING SETS [, ...] (  groupSet [ , groupSet [ , ... ] ] ) |
ROLLUP(expr  [ , expr [ , ... ] ]) |
expr  [ , expr [ , ... ] ] WITH ROLLUP |
CUBE(expr  [ , expr [ , ... ] ]) |
expr  [ , expr [ , ... ] ] WITH CUBE
]
[ ... ]``````

### Parameters

`groupSet` represents a set composed of columns, aliases or expressions in the select list. `groupSet ::= { ( expr [ , expr [ , ... ] ] )}`

`expr` indicates the column, alias or expression in the select list.

### Note

starrocks supports syntax like PostgreSQL. The syntax examples are as follows:

``````  SELECT a, b, SUM( c ) FROM tab1 GROUP BY GROUPING SETS ( (a, b), (a), (b), ( ) );
SELECT a, b,c, SUM( d ) FROM tab1 GROUP BY ROLLUP(a,b,c)
SELECT a, b,c, SUM( d ) FROM tab1 GROUP BY CUBE(a,b,c)``````

`ROLLUP(a,b,c)` is equivalent to the following`GROUPING SETS` statement:

``````  GROUPING SETS (
(a,b,c),
( a, b ),
( a),
( )
)``````

`CUBE ( a, b, c )` is equivalent to the following`GROUPING SETS` statement:

``````  GROUPING SETS (
( a, b, c ),
( a, b ),
( a,    c ),
( a       ),
(    b, c ),
(    b    ),
(       c ),
(         )
)``````

## Examples

The following is an example of actual data:

``````  > SELECT * FROM t;
+------+------+------+
| k1   | k2   | k3   |
+------+------+------+
| a    | A    |    1 |
| a    | A    |    2 |
| a    | B    |    1 |
| a    | B    |    3 |
| b    | A    |    1 |
| b    | A    |    4 |
| b    | B    |    1 |
| b    | B    |    5 |
+------+------+------+
8 rows in set (0.01 sec)

> SELECT k1, k2, SUM(k3) FROM t GROUP BY GROUPING SETS ( (k1, k2), (k2), (k1), ( ) );
+------+------+-----------+
| k1   | k2   | sum(`k3`) |
+------+------+-----------+
| b    | B    |         6 |
| a    | B    |         4 |
| a    | A    |         3 |
| b    | A    |         5 |
| NULL | B    |        10 |
| NULL | A    |         8 |
| a    | NULL |         7 |
| b    | NULL |        11 |
| NULL | NULL |        18 |
+------+------+-----------+
9 rows in set (0.06 sec)

> SELECT k1, k2, GROUPING_ID(k1,k2), SUM(k3) FROM t GROUP BY GROUPING SETS ((k1, k2), (k1), (k2), ());
+------+------+---------------+----------------+
| k1   | k2   | grouping_id(k1,k2) | sum(`k3`) |
+------+------+---------------+----------------+
| a    | A    |             0 |              3 |
| a    | B    |             0 |              4 |
| a    | NULL |             1 |              7 |
| b    | A    |             0 |              5 |
| b    | B    |             0 |              6 |
| b    | NULL |             1 |             11 |
| NULL | A    |             2 |              8 |
| NULL | B    |             2 |             10 |
| NULL | NULL |             3 |             18 |
+------+------+---------------+----------------+
9 rows in set (0.02 sec)``````
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