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

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# count {#agg_function-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](../../../sql-reference/syntax.md#syntax-expressions).
**Returned value**
- If the function is called without parameters it counts the number of rows.
- If the [expression](../../../sql-reference/syntax.md#syntax-expressions) is passed, then the function counts how many times this expression returned not null. If the expression returns a [Nullable](../../../sql-reference/data-types/nullable.md)-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](../../../sql-reference/data-types/int-uint.md).
**Details**
2020-10-13 17:23:29 +00:00
ClickHouse supports the `COUNT(DISTINCT ...)` syntax. The behavior of this construction depends on the [count_distinct_implementation](../../../operations/settings/settings.md#settings-count_distinct_implementation) setting. It defines which of the [uniq\*](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) functions is used to perform the operation. The default is the [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-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.
**Examples**
Example 1:
``` sql
SELECT count() FROM t
```
``` text
┌─count()─┐
│ 5 │
└─────────┘
```
Example 2:
``` sql
SELECT name, value FROM system.settings WHERE name = 'count_distinct_implementation'
```
``` text
┌─name──────────────────────────┬─value─────┐
│ count_distinct_implementation │ uniqExact │
└───────────────────────────────┴───────────┘
```
``` sql
SELECT count(DISTINCT num) FROM t
```
``` text
┌─uniqExact(num)─┐
│ 3 │
└────────────────┘
```
This example shows that `count(DISTINCT num)` is performed by the `uniqExact` function according to the `count_distinct_implementation` setting value.