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 where student.sno=score.sno ----当查询的列名两个表中都有时要在列名前面加上‘表名.’
法2：select student.sno ,sname,cno,degree from student join score on student.sno=score.sno
--- //inner join(默认) //left join（以左表为主表） //right join（以右表为主表）
3、多个表的连接
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 where student.sno=score.sno and score.cno=course.cno
----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.’
法2：select student.sno, sname, cno, degree, name from student join score on student.sno=course.sn join course on score.cno=course.cno
4、纵链接
unine