我对外连接的理解

mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 |
+-------+--------+-----------+------+------------+---------+---------+--------+

1. 内连接：选出所有有领导的员工

select e1.ename,e2.ename
from emp e1
join emp e2
on e1.empno=e2.mgr
+-------+--------+
| ename | ename  |
+-------+--------+
| FORD  | SMITH  |
| BLAKE | ALLEN  |
| BLAKE | WARD   |
| KING  | JONES  |
| BLAKE | MARTIN |
| KING  | BLAKE  |
| KING  | CLARK  |
| JONES | SCOTT  |
| BLAKE | TURNER |
| SCOTT | ADAMS  |
| BLAKE | JAMES  |
| JONES | FORD   |
| CLARK | MILLER |
+-------+--------+

2. 左连接：选出所有领导，即使他的下属是null

select e1.ename,e2.ename
from emp e1
left join emp e2
on e1.empno=e2.mgr
相当于
select e1.ename,e2.ename
from emp e1
join emp e2
on e1.empno=e2.mgr or e1.empno=null.mgr#虽然这么写不对，但可以这么理解
+--------+--------+
| ename  | ename  |
+--------+--------+
| SMITH  | NULL   |
| ALLEN  | NULL   |
| WARD   | NULL   |
| JONES  | SCOTT  |
| JONES  | FORD   |
| MARTIN | NULL   |
| BLAKE  | ALLEN  |
| BLAKE  | WARD   |
| BLAKE  | MARTIN |
| BLAKE  | TURNER |
| BLAKE  | JAMES  |
| CLARK  | MILLER |
| SCOTT  | ADAMS  |
| KING   | JONES  |
| KING   | BLAKE  |
| KING   | CLARK  |
| TURNER | NULL   |
| ADAMS  | NULL   |
| JAMES  | NULL   |
| FORD   | SMITH  |
| MILLER | NULL   |
+--------+--------+

3. 右连接：选出所有下属，即使他的领导是null

select e1.ename,e2.ename
from emp e1
right join emp e2
on e1.empno=e2.mgr
相当于
select e1.ename,e2.ename
from emp e1
join emp e2
on e1.empno=e2.mgr or null.empno=e2.mgr#虽然这么写不对，但可以这么理解
+-------+--------+
| ename | ename  |
+-------+--------+
| FORD  | SMITH  |
| BLAKE | ALLEN  |
| BLAKE | WARD   |
| KING  | JONES  |
| BLAKE | MARTIN |
| KING  | BLAKE  |
| KING  | CLARK  |
| JONES | SCOTT  |
| NULL  | KING   |
| BLAKE | TURNER |
| SCOTT | ADAMS  |
| BLAKE | JAMES  |
| JONES | FORD   |
| CLARK | MILLER |
+-------+--------+