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ClickHouse/docs/en/sql-reference/statements/select/where.md
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[docs] split aggregate function and system table references (#11742) 
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WHERE

WHERE Clause

WHERE clause allows to filter the data that is coming from FROM 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.