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 excluded 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

There’s a filtering optimization called prewhere.