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

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count

Counts the number of rows or not-NULL values.

ClickHouse supports the following syntaxes for count:

• count(expr) or COUNT(DISTINCT expr).
• count() or COUNT(*). The count() syntax is ClickHouse-specific.

Arguments

The function can take:

• Zero parameters.
• One expression.

Returned value

• If the function is called without parameters it counts the number of rows.
• If the expression is passed, then the function counts how many times this expression returned not null. If the expression returns a Nullable-type value, then the result of count stays not Nullable. The function returns 0 if the expression returned NULL for all the rows.

In both cases the type of the returned value is UInt64.

Details

ClickHouse supports the COUNT(DISTINCT ...) syntax. The behavior of this construction depends on the count_distinct_implementation setting. It defines which of the uniq* functions is used to perform the operation. The default is the uniqExact function.

The SELECT count() FROM table query is not optimized, because the number of entries in the table is not stored separately. It chooses a small column from the table and counts the number of values in it.

However SELECT count(nullable_column) FROM table query can be optimized by enabling the optimize_functions_to_subcolumns setting. With optimize_functions_to_subcolumns = 1 the function reads only null subcolumn instead of reading and processing the whole column data. The query SELECT count(n) FROM table transforms to SELECT sum(NOT n.null) FROM table.

Examples

Example 1:

SELECT count() FROM t

┌─count()─┐
│       5 │
└─────────┘

Example 2:

SELECT name, value FROM system.settings WHERE name = 'count_distinct_implementation'

┌─name──────────────────────────┬─value─────┐
│ count_distinct_implementation │ uniqExact │
└───────────────────────────────┴───────────┘

SELECT count(DISTINCT num) FROM t

┌─uniqExact(num)─┐
│              3 │
└────────────────┘

This example shows that count(DISTINCT num) is performed by the uniqExact function according to the count_distinct_implementation setting value.