表连接分为横向表连接和纵向表连接
横向表连接有三种方式：
1、select 列名，列名 from 表名，表名 where 表名.列名=表名.列名
select student.Sno,sname,cno,degree from Student,Score
where Student.Sno = Score.Sno
2、子查询方式
select sno,sname,(select cno from Score where Student.Sno=Score.Sno) from Student
3、select 列名，列名 from 表名 join 表名 on 表名.列名=表名.列名
select Student.Sno,sname,Score.Cno,DEGREE,cname
from Student join Score on Student.Sno=Score.Sno
join Course on Score.Cno = Course.cno
纵向表连接：（注：两个表必须是同样的列数，对应的列数据类型必须一致）
select 列名，列名 from 表名 union select 列名，列名 from 表名
select sno 编号,sname 姓名,ssex 性别,sbirthday 生日 from Student
union
select tno,tname,tsex,tbirthday from Teacher
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灵活用法
select Student.sno,sname+'同学',cname,abs(degree+10)
from Student,Score,Course
where Student.Sno=Score.Sno and Score.Cno=Course.Cno
and student.Sno='107'