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1.6 KiB
1.6 KiB
Aggregate functions
Aggregate functions work in the normal way as expected by database experts.
ClickHouse also supports:
- Parametric aggregate functions, which accept other parameters in addition to columns.
- Combinators, which change the behavior of aggregate functions.
NULL processing
During aggregation, all NULLs are skipped.
Examples:
Consider this table:
┌─x─┬────y─┐
│ 1 │ 2 │
│ 2 │ ᴺᵁᴸᴸ │
│ 3 │ 2 │
│ 3 │ 3 │
│ 3 │ ᴺᵁᴸᴸ │
└───┴──────┘
Let's say you need to total the values in the y column:
:) SELECT sum(y) FROM t_null_big
SELECT sum(y)
FROM t_null_big
┌─sum(y)─┐
│ 7 │
└────────┘
1 rows in set. Elapsed: 0.002 sec.
The sum function interprets NULL as 0. In particular, this means that if the function receives input of a selection where all the values are NULL, then the result will be 0, not NULL.
Now you can use the groupArray function to create an array from the y column:
:) SELECT groupArray(y) FROM t_null_big
SELECT groupArray(y)
FROM t_null_big
┌─groupArray(y)─┐
│ [2,2,3] │
└───────────────┘
1 rows in set. Elapsed: 0.002 sec.
groupArray does not include NULL in the resulting array.