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
There's a filtering optimization called prewhere.