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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.