三表连接之内连接

Posted 2020-10-01 长大当个科学家

表之间的关系为：

manager 1-------------->n department 1----------------->n employee

mysql> select * from manager;

 

2 rows in set (0.00 sec)

mysql> select * from department;

 

3 rows in set (0.00 sec)

mysql> select * from employee;

 

2 rows in set (0.00 sec)

mysql> select * from manager as ma inner join department as de on ma.id=de.manag
er_id inner join employee as em on de.id=department_id;

 

2 rows in set (0.02 sec)

三张表的连接可理解为这样：
mysql> select * from manager as ma inner join department as de on ma.id=de.manag
er_id;

 

先让manager和department两张表做内连接，结果如上

然后再用这张表与employee做内连接。结果如下

mysql> select * from manager as ma inner join department as de on ma.id=de.manag
er_id inner join employee as em on de.id=department_id;