表连接
1、select * from student，score ——笛卡尔积
2、两个表的连接：
法1：select student.sno, sname, degree
from student,score ----当查询的列名两个表中都有时要在列名前面加上‘表名.’
where student.sno=score.sno
法2：select cno, (select sname from student where student.sno=score.sno),degree from score
法3：select student.sno ,sname,cno,degree
from student join score --- //inner join(默认) //left join（以左表为主表） //right join（以右表为主表）
on student.sno=score.sno
3、三个表的连接
法1：
select student.sno, sname, cno, degree, name ---注：select student.sno, sname, cno, degree+10, name+‘同学’ 结果是：成绩增加10分，名字后面加上同学
from student,score，course ----当查询的列名不只存在1个表中都有时要在列名前面加上‘表名.’
where student.sno=score.sno
and score.cno=course.cno
法2：select student.sno, sname, cno, degree, name
from student join score
on student.sno=course.sno
join course
on score.cno=course.cno
4、纵链接