编辑代码

CREATE TABLE T (
    Tno INT PRIMARY KEY,
    Title ENUM('讲师', '副教授', '教授')
);
CREATE TABLE C (
    Cno INT PRIMARY KEY,
    Tno INT,
    FOREIGN KEY (Tno) REFERENCES T(Tno)
);
ALTER TABLE C ADD CONSTRAINT uc_Cname UNIQUE (Cname);
ALTER TABLE C ADD COLUMN Credit INT;
ALTER TABLE C DROP COLUMN Credit;
ALTER TABLE C DROP INDEX uc_Cname;
CREATE TABLE SC (
    Sno INT,
    Cno INT,
    PRIMARY KEY (Sno, Cno), -- 复合主键实现实体完整性
    FOREIGN KEY (Cno) REFERENCES C(Cno), -- 参照完整性，参照C表的Cno
    CHECK (Sno > 0) -- 用户定义完整性，假设要求学生编号大于0
);