老师说：CREATE TABLE Student
(Sno CHAR(9) PRIMARY KEY, /* 列级完整性约束条件*/
Sname CHAR(20) UNIQUE, /* Sname取唯一值*/
Ssex CHAR(2),
Sage SMALLINT,
Sdept CHAR(20)
)
CREATE TABLE Course
( Cno CHAR(4) PRIMARY KEY,
Cname CHAR(40),
Cpno CHAR(4) ,
Ccredit SMALLINT,
FOREIGN KEY (Cpno) REFERENCES Course(Cno)
);
CREATE TABLE SC
(Sno CHAR(9),
Cno CHAR(4),
Grade SMALLINT,
PRIMARY KEY (Sno,Cno),
/* 主码由两个属性构成，必须作为表级完整性进行定义*/
FOREIGN KEY (Sno) REFERENCES Student(Sno),
/* 表级完整性约束条件，Sno是外码，被参照表是Student */
FOREIGN KEY (Cno) REFERENCES Course(Cno)
/* 表级完整性约束条件， Cno是外码，被参照表是Course*/



);