CASE can be used in any statement or clause that allows a valid expression. For example, you can use CASE in statements such as SELECT, UPDATE, DELETE and SET, and in clauses such as <select_list>, IN, WHERE, ORDER BY, and HAVING.

CASE Expression in SQL
The CASE expression checks conditions and returns a value when the first condition is met (similar to an if-then-else statement). As a result, if a condition is met, it will finish reading and return the result. If none of the conditions are met, it returns the value specified in the ELSE clause.