ClickHouse/docs/en/sql-reference/statements/select/where.md

# 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).