SQL server 数据库——表连接（多表横向连接，纵向连接）

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

巴特西