数据库原理：数据库系统原理-第3章(2)

CREARE TABLE SC (SNO VARCHAR(8) NOT NULL, CNO VARCHAR(6) NOT NULL, GRADE DEC(4,1) DEFAULT NULL, PRIMARY KEY (SNO,CNO), FOREIGN KEY (SNO) REFERENCES STUDENT ON DELETE ? FOREIGN KEY (CNO) REFERENCES COURSE ON DELETE ? CHECK (GRADE IS NULL) OR (GRADE BETWEEN 0 AND 100.0) )