ClickHouse/docs/en/query_language/agg_functions/index.md

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

┌─sum(y)─┐
│      7 │
└────────┘

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

┌─groupArray(y)─┐
│ [2,2,3]       │
└───────────────┘

groupArray does not include NULL in the resulting array.

Original article