sql语句札记

组合查询：

sql的组合查询操作符有UNION（并操作),INTERSECT(交操作)和MINUS(差操作)3种

1.union操作与条件or等价

2.interset不与and等价

3.minus例子：求选修了c1课程但没有选修c2课程的学生的学号
select xuhao from xuanke where kch='c1'
minus
select xuhao from xuanke
where kch='c2'

等价于

select xuehao from xuanke x1
where kch='c1' and not exists
(select xh from xuanke xk2
where xk1.xh=xk2.xh and xk2.kch='c2');