mysql求交集：UNION ALL合并查询，inner join内连接查询，IN/EXISTS子查询

1. 两个要求交集的表（列）的结构要一致,对应的字段数,字段类型都应该相同；将两个数据的数据列用 UNION ALL 关键字合并；将上面的所有需要比较的列 GROUP BY ；最后 HAVING COUNT(任意一列，不能多列)>1,那么就是交集。

   SELECT a.* FROM(    SELECT * from teacher    UNION ALL    SELECT * from student)a GROUP BY a.id,a.name,a.sex HAVING COUNT(a.sex)>1

    

2. 内连接inner join。将两个数据列，条件必须涉及需要比较的列，这里没有比较sex列，所以比2步多了一行数据。

   SELECT * FROM student AS a JOIN teacher AS b ON a.name =b.name AND a.ID=b.ID或者SELECT * FROM student AS a inner  JOIN teacher AS b ON a.name =b.name AND a.ID=b.ID

    
3. 带IN关键字的查询。需要比较多少列数据是相同的就必须有多少个IN。这个时候一定要控制列，列太多不建议用。

   SELECT * FROM student AS a where a.name in (select name from teacher )AND a.ID in (select ID from teacher);

    
4. 带EXISTS关键字的子查询。这个语句说白了也是通过判断teacher中是否有满足两个相等的条件，来得出来的。SELECT * FROM student AS a where EXISTS (select *from teacher b where a.name =b.name AND a.ID=b.ID)