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1.子查询

(1)from子查询

select t.grade,count(t.empno) 员工数 from (select * from salgrade join emp on emp.sal between losal and hisal) t group by grade;

（2）在连接的表中查询一列或几列数据时，需要带上相应的表名

select t1.ename,t2.dname,t2.部门人数 from(select ename,deptno from emp where job='manager') t1 join (select dept.deptno,dept.dname,count(emp.empno) 部门人数 from dept join emp on dept.deptno=emp.deptno group by dept.deptno) t2 on t1.deptno=t2.deptno;

2.自连接

select *

from emp as 领导表

join emp as 员工表 on 领导表.empno=员工表.mgr

join dept on 员工表.deptno=dept.deptno

where 员工表.hiredate<领导表.hiredate;#老师思路