GROUPING

This function is particularly useful when working with GROUPING SETS, ROLLUP or CUBE clauses.

GROUPING SETS, ROLLUP, and CUBE clauses produce aggregates that use NULL values to pad columns that are not part of the current grouping set. This may cause ambiguity if those columns also contain actual NULL values. The GROUPING function helps resolve this ambiguity by distinguishing between columns that are excluded from a grouping set and those that genuinely contain NULL values.

Key points about the GROUPING function:

  • It accepts expressions that are also used in the GROUP BY [GROUPING SETS | ROLLUP | CUBE] clause as arguments.
  • It returns an integer where each bit represents whether the corresponding column is part of the grouping set or not.
  • A bit is set to 1 if the corresponding column is not part of the grouping set; otherwise, it is 0.
  • The first argument corresponds to the most significant bit, with subsequent arguments following in order.

Syntax 

GROUPING(<expression> [, ...n])

Parameters

Parameter Description Supported input types
<expression> An expression that also appears as a <grouping_element> in a GROUPING SETS, ROLLUP, or CUBE clause. The maximum number of arguments is 31. Any

Return Type

GROUPING returns a value of type INT.

Examples

Consider this table addresses:

user_id country county city
3 USA California Los Angeles
34 USA California Los Angeles
15 Canada NULL NULL
1 Canada Quebec Montreal
45 Canada Quebec Montreal
10 USA California Springfield
11 USA Oregon Springfield
23 Canada Ontario London
30 UK Greater London London

Let’s say we are interested in the number of users per region, going from the city level up to country level:

select 
    grouping(country, county, city), 
    country, 
    county, 
    city, 
    count(*)
from 
    locations
group by 
    grouping sets (
        (country, county, city),
        (country, county),
        (country)
    )
order by
    1, 2, 3, 4

The query returns this result:

grouping country county city count
0 Canada Ontario London 1
0 Canada Quebec Montreal 2
0 Canada null null 1
0 UK Greater London London 1
0 USA California Los Angeles 2
0 USA California Springfield 1
0 USA Oregon Springfield 1
1 Canada Ontario null 1
1 Canada Quebec null 2
1 Canada null null 1
1 UK Greater London null 1
1 USA California null 3
1 USA Oregon null 1
3 Canada null null 4
3 UK null null 1
3 USA null null 4

The GROUPING function evaluates to three distinct values: 0, 1, and 3. Each value corresponds to a specific grouping set defined in the GROUP BY GROUPING SETS clause.

  • A value of 0 indicates that the row is grouped by all specified columns.
  • A value of 1 (0b001 in binary) signifies that the city column is excluded from the grouping.
  • A value of 3 (0b011 in binary) indicates that both the county and city columns are excluded from the grouping.

This allows us to differentiate between rows with the same grouping key values, such as Canada, NULL, NULL, by identifying which columns were used for aggregation in each case.