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78 lines
3.5 KiB
Markdown
78 lines
3.5 KiB
Markdown
---
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slug: /en/sql-reference/aggregate-functions/reference/count
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sidebar_position: 120
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---
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# count
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Counts the number of rows or not-NULL values.
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ClickHouse supports the following syntaxes for `count`:
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- `count(expr)` or `COUNT(DISTINCT expr)`.
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- `count()` or `COUNT(*)`. The `count()` syntax is ClickHouse-specific.
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**Arguments**
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The function can take:
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- Zero parameters.
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- One [expression](../../../sql-reference/syntax.md#syntax-expressions).
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**Returned value**
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- If the function is called without parameters it counts the number of rows.
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- 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.
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In both cases the type of the returned value is [UInt64](../../../sql-reference/data-types/int-uint.md).
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**Details**
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ClickHouse supports the `COUNT(DISTINCT ...)` syntax. The behavior of this construction depends on the [count_distinct_implementation](../../../operations/settings/settings.md#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.
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The `SELECT count() FROM table` query is optimized by default using metadata from MergeTree. If you need to use row-level security, disable optimization using the [optimize_trivial_count_query](../../../operations/settings/settings.md#optimize-trivial-count-query) setting.
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However `SELECT count(nullable_column) FROM table` query can be optimized by enabling the [optimize_functions_to_subcolumns](../../../operations/settings/settings.md#optimize-functions-to-subcolumns) setting. With `optimize_functions_to_subcolumns = 1` the function reads only [null](../../../sql-reference/data-types/nullable.md#finding-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`.
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**Improving COUNT(DISTINCT expr) performance**
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If your `COUNT(DISTINCT expr)` query is slow, consider adding a [`GROUP BY`](../../../sql-reference/statements/select/group-by.md) clause as this improves parallelization. You can also use a [projection](../../../sql-reference/statements/alter/projection.md) to create an index on the target column used with `COUNT(DISTINCT target_col)`.
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**Examples**
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Example 1:
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``` sql
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SELECT count() FROM t
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```
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``` text
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┌─count()─┐
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│ 5 │
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└─────────┘
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```
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Example 2:
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``` sql
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SELECT name, value FROM system.settings WHERE name = 'count_distinct_implementation'
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```
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``` text
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┌─name──────────────────────────┬─value─────┐
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│ count_distinct_implementation │ uniqExact │
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└───────────────────────────────┴───────────┘
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```
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``` sql
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SELECT count(DISTINCT num) FROM t
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```
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``` text
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┌─uniqExact(num)─┐
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│ 3 │
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└────────────────┘
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```
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This example shows that `count(DISTINCT num)` is performed by the `uniqExact` function according to the `count_distinct_implementation` setting value.
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