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

There is an exception to this rule, which are the functions first_value, last_value and their aliases (any and anyLast respectively) when followed by the modifier RESPECT NULLS. For example, FIRST_VALUE(b) RESPECT NULLS.

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. 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]                     │
└───────────────┴───────────────────────────────────────┘

Note that aggregations are skipped when the columns are used as arguments to an aggregated function. For example count without parameters (count()) or with constant ones (count(1)) will count all rows in the block (independently of the value of the GROUP BY column as it's not an argument), while count(column) will only return the number of rows where column is not NULL.

SELECT
    v,
    count(1),
    count(v)
FROM
(
    SELECT if(number < 10, NULL, number % 3) AS v
    FROM numbers(15)
)
GROUP BY v

┌────v─┬─count()─┬─count(v)─┐
│ ᴺᵁᴸᴸ │      10 │        0 │
│    0 │       1 │        1 │
│    1 │       2 │        2 │
│    2 │       2 │        2 │
└──────┴─────────┴──────────┘

And here is an example of first_value with RESPECT NULLS where we can see that NULL inputs are respected and it will return the first value read, whether it's NULL or not:

SELECT
    col || '_' || ((col + 1) * 5 - 1) as range,
    first_value(odd_or_null) as first,
    first_value(odd_or_null) IGNORE NULLS as first_ignore_null,
    first_value(odd_or_null) RESPECT NULLS as first_respect_nulls
FROM
(
    SELECT
        intDiv(number, 5) AS col,
        if(number % 2 == 0, NULL, number) as odd_or_null
    FROM numbers(15)
)
GROUP BY col
ORDER BY col

┌─range─┬─first─┬─first_ignore_null─┬─first_respect_nulls─┐
│ 0_4   │     1 │                 1 │                ᴺᵁᴸᴸ │
│ 1_9   │     5 │                 5 │                   5 │
│ 2_14  │    11 │                11 │                ᴺᵁᴸᴸ │
└───────┴───────┴───────────────────┴─────────────────────┘