创建三个表

CREATE TABLE Student (
Sno char(7) PRIMARY KEY,
--在列级完整性约束处定义主码约束
Sname char(10) NOT NULL,
--非空约束
Ssex char(2),
Sage tinyint,
Sdept char(20)
)
CREATE TABLE Course(
Cno char(6) NOT NULL,
Cname char(20) NOT NULL,
Credit tinyint,
Semester tinyint,
PRIMARY KEY(Cno)
--在表级完整性约束处定义主码约束
)
CREATE TABLE SC(
Sno char(7) NOT NULL,
Cno char(6) NOT NULL,
Grade smallint,
PRIMARY KEY(Sno,Cno),
FOREIGN KEY(Sno) REFERENCES Student(Sno),
FOREIGN KEY(Cno) REFERENCES Course(Cno)
)