表之間的關系為：

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)

三張表的連接可理解為這樣：
mysql> select * from manager as ma inner join department as de on ma.id=de.manag
er_id;

先讓manager和department兩張表做內連接，結果如上

然后再用這張表與employee做內連接。結果如下

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;

總結

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