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

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groupArray

Syntax: groupArray(x) or groupArray(max_size)(x)

Creates an array of argument values. Values can be added to the array in any (indeterminate) order.

The second version (with the max_size parameter) limits the size of the resulting array to max_size elements. For example, groupArray(1)(x) is equivalent to [any (x)].

In some cases, you can still rely on the order of execution. This applies to cases when SELECT comes from a subquery that uses ORDER BY.

Example show default.ck table result.

┌─id─┬─name─────┐
│  1 │ zhangsan │
│  1 │ ᴺᵁᴸᴸ     │
│  1 │ lisi     │
│  2 │ wangwu   │
└────┴──────────┘

Query:

select id, groupArray(10)(name) from(select id,name from  default.ck group by id,name order by id) group by id;

Result:

┌─id─┬─groupArray(10)(name)─┐
│  1 │ ['zhangsan','lisi']  │
│  2 │ ['wangwu']           │
└────┴──────────────────────┘

The groupArray function will remove ᴺᵁᴸᴸ value based on the above results.