Correct the query

CREATE OR REPLACE FUNCTION EMPLOYEE_AFTER_1968() returns bigint as $date$ declare birthdate bigint; BEGIN SELECT birthdate FROM EMPLOYEE WHERE BIRTHDATE >= DATE('1968-01-01', '00:00:00'); return date; END; $date$ language plpgsql

Message SQL Error [42702]: ERROR: column reference "birthdate" A is ambiguous Detail: It could refer to either a PL/pgSQL variable or a table column.1 Where: PL/pgSQL function employee_after_19680 line 4 at SQL statement