create table students( id varchar(32) primary key, name varchar(30) );

create table course( id varchar(32) primary key, name varchar(30) );

create table sc( studid varchar(32), courseid varchar(32), constraint sc_pk primary key(studid,courseid), constraint sc_fk1 foreign key(studid) REFERENCES students(id), constraint sc_fk2 foreign key(courseid) references course(id) );

写入记录： insert into students values('S001','Jack'); insert into students values('S002','Mary'); insert into students values('S003','Alex');

insert into course values('C001','java'); insert into course values('C002','Oracle'); insert into course values('C003','Math');

insert into sc values('S001','C001'); insert into sc values('S001','C002'); insert into sc values('S002','C001');

1:查询某个选择了某课： select s.name,c.name from students s inner join sc on s.id=sc.studid inner join COURSE c on sc.COURSEID=c.id;