多表联查

1. 内连接查询
2. 外连接查詢
3. 查询结果合并
4. 子查询

建表语句：
drop table if exists t_dept;
create table t_dept(
_id int primary key,
deptno int(11),
dname varchar(20),
loc varchar(50));

插入数据语句：
insert into t_dept
(_id,deptno,dname,loc)
values
('1','10','ACCOUNTING','NEW YORK'),
('2','20','RESEARCH','DALLAS'),
('3','30','SALES','CHICAGO'),
('4','40','OPERATIONS','BOSTON')

1.内连接

方法1：
select t1.ename,t1.job,t2.ename 
from
      t_employee t1,
      t_employee t2 
where
        t1.mgr = t2.empno; 


方法2：
select t1.ename,t1.job,t2.ename
from t_employee t1
inner join t_employee t2 on t1.mgr = t2.empno;

2.外链接查询

左外连接,left左边作为主表
select t1.ename,t1.job,t2.ename as mgr_name
from t_employee t1
left outer  t_employee t2 on t1.mgr = t2.empno;

右外连接
select t1.empno,t1.ename,t1.job,t2.dname,t2.loc
from t_employee t1
right outer  t_dept t2 on t1.deptno = t2.deptno;