ClickHouse/docs/en/sql-reference/aggregate-functions/index.md

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/en/sql-reference/aggregate-functions/	Aggregate Functions	33

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. If the aggregation has several parameters it will ignore any row in which one or more of the parameters are NULL.

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 │
└────────┘

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.

You can use COALESCE to change NULL into a value that makes sense in your use case. For example: avg(COALESCE(column, 0)) with use the column value in the aggregation or zero if NULL:

SELECT
    avg(y),
    avg(coalesce(y, 0))
FROM t_null_big

┌─────────────avg(y)─┬─avg(coalesce(y, 0))─┐
│ 2.3333333333333335 │                 1.4 │
└────────────────────┴─────────────────────┘

Also you can use Tuple to work around NULL skipping behavior. The a Tuple that contains only a NULL value is not NULL, so the aggregate functions won't skip that row because of that NULL value.

SELECT
    groupArray(y),
    groupArray(tuple(y)).1
FROM t_null_big;

┌─groupArray(y)─┬─tupleElement(groupArray(tuple(y)), 1)─┐
│ [2,2,3]       │ [2,NULL,2,3,NULL]                     │
└───────────────┴───────────────────────────────────────┘