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58 lines
1.9 KiB
Markdown
58 lines
1.9 KiB
Markdown
---
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sidebar_label: WHERE
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---
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# WHERE Clause
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`WHERE` clause allows to filter the data that is coming from [FROM](../../../sql-reference/statements/select/from.md) clause of `SELECT`.
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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.
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`WHERE` expression is evaluated on the ability to use indexes and partition pruning, if the underlying table engine supports that.
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:::note
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There is a filtering optimization called [PREWHERE](../../../sql-reference/statements/select/prewhere.md).
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:::
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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.
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Otherwise an expression with `NULL` never passes.
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**Example**
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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):
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``` sql
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SELECT number FROM numbers(20) WHERE (number > 10) AND (number % 3 == 0);
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```
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Result:
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``` text
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┌─number─┐
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│ 12 │
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│ 15 │
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│ 18 │
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└────────┘
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```
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Queries with `NULL` values:
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``` sql
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CREATE TABLE t_null(x Int8, y Nullable(Int8)) ENGINE=MergeTree() ORDER BY x;
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INSERT INTO t_null VALUES (1, NULL), (2, 3);
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SELECT * FROM t_null WHERE y IS NULL;
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SELECT * FROM t_null WHERE y != 0;
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```
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Result:
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``` text
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┌─x─┬────y─┐
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│ 1 │ ᴺᵁᴸᴸ │
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└───┴──────┘
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┌─x─┬─y─┐
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│ 2 │ 3 │
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└───┴───┘
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```
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