ClickHouse/docs/en/sql-reference/statements/select/where.md
2022-04-09 07:34:21 -06:00

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---
sidebar_label: WHERE
---
# WHERE Clause {#select-where}
`WHERE` clause allows to filter the data that is coming from [FROM](../../../sql-reference/statements/select/from.md) clause of `SELECT`.
If there is a `WHERE` clause, it must contain an expression with the `UInt8` type. This is usually an expression with comparison and logical operators. Rows where this expression evaluates to `0` are excluded from further transformations or result.
`WHERE` expression is evaluated on the ability to use indexes and partition pruning, if the underlying table engine supports that.
:::note
There is a filtering optimization called [PREWHERE](../../../sql-reference/statements/select/prewhere.md).
:::
If you need to test a value for [NULL](../../../sql-reference/syntax.md#null-literal), use [IS NULL](../../operators/index.md#operator-is-null) and [IS NOT NULL](../../operators/index.md#is-not-null) operators or [isNull](../../../sql-reference/functions/functions-for-nulls.md#isnull) and [isNotNull](../../../sql-reference/functions/functions-for-nulls.md#isnotnull) functions.
Otherwise an expression with `NULL` never passes.
**Example**
To find numbers that are multiples of 3 and are greater than 10 execute the following query on the [numbers table](../../../sql-reference/table-functions/numbers.md):
``` sql
SELECT number FROM numbers(20) WHERE (number > 10) AND (number % 3 == 0);
```
Result:
``` text
┌─number─┐
│ 12 │
│ 15 │
│ 18 │
└────────┘
```
Queries with `NULL` values:
``` sql
CREATE TABLE t_null(x Int8, y Nullable(Int8)) ENGINE=MergeTree() ORDER BY x;
INSERT INTO t_null VALUES (1, NULL), (2, 3);
SELECT * FROM t_null WHERE y IS NULL;
SELECT * FROM t_null WHERE y != 0;
```
Result:
``` text
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
└───┴──────┘
┌─x─┬─y─┐
│ 2 │ 3 │
└───┴───┘
```