4. --A
ALTER TABLE classesTAU
ADD (class_floor number(3));
-- B
ALTER TABLE classesTAU
MODIFY (class_floor varchar2(10);
-- C
ALTER TABLE classesTAU
ADD (CONSTRAINT clas_fl_ck CHECK(class_floor IN ('A1' , 'A2' , 'A3') OR
class_floor IS NULL))
-- D
ALTER TABLE classesTAU
DROP CONSTRAINT clas_fl_ck
-- E
ALTER TABLE classesTAU
DROP COLUMN class_floor