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15 lines
705 B
Markdown
15 lines
705 B
Markdown
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toc_title: WHERE
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---
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# WHERE Clause {#select-where}
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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 expluded 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 "Note"
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There’s a filtering optimization called [prewhere](../../../sql-reference/statements/select/prewhere.md).
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