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toc_title: WHERE
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# WHERE Clause {#select-where}

`WHERE` clause allows to filter the data that is coming from [FROM](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 expluded 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 "Note"
    There's a filtering optimization called [prewhere](prewhere.md).