The IN operator checks containment of the left expression inside the set of expressions on the right hand side (RHS). The IN operator returns true if the expression is present in the RHS, false if the expression is not in the RHS and the RHS has no NULL values, or NULL if the expression is not in the RHS and the RHS has NULL values.

SELECT 'Math' IN ('CS', 'Math');
-- true
SELECT 'English' IN ('CS', 'Math');
-- false

SELECT 'Math' IN ('CS', 'Math', NULL);
-- true
SELECT 'English' IN ('CS', 'Math', NULL);
-- NULL

NOT IN can be used to check if an element is not present in the set. X NOT IN Y is equivalent to NOT(X IN Y).

The IN operator can also be used with a subquery that returns a single column. See the subqueries page for more information.