//本文使用的數據表格

//persons表中id_p為主鍵

//orders表中id_o為主鍵，id_p為外鍵參考persons表中的id_p

mysql> select * from persons;select * fromorders;+------+----------+-----------+----------------+----------+

| id_p | lastname | firstname | address | city |

+------+----------+-----------+----------------+----------+

| 1 | Adams | John | Oxford Street | London |

| 2 | Bush | George | Fifth Avenue | New York |

| 3 | Carter | Thomas | Changan Street | Beijing |

+------+----------+-----------+----------------+----------+

3 rows in set (0.05sec)+------+---------+------+

| id_o | orderNo | id_p |

+------+---------+------+

| 1 | 77895 | 3 |

| 2 | 44678 | 3 |

| 3 | 22456 | 1 |

| 4 | 24562 | 1 |

| 5 | 34764 | 65 |

+------+---------+------+

5 rows in set (0.05 sec)

1.內連接 [inner] join：

內連接也叫連接，或者自然連接：

//inner可刪

mysql> select lastname,firstname,orderNo from persons inner join orders on persons.id_p=orders.id_p order by lastname;+----------+-----------+---------+

| lastname | firstname | orderNo |

+----------+-----------+---------+

| Adams | John | 24562 |

| Adams | John | 22456 |

| Carter | Thomas | 77895 |

| Carter | Thomas | 44678 |

+----------+-----------+---------+

//使用where達到同樣的效果

mysql> select lastname,firstname,orderNo from persons,orders where persons.id_p=orders.id_p;+----------+-----------+---------+

| lastname | firstname | orderNo |

+----------+-----------+---------+

| Carter | Thomas | 77895 |

| Carter | Thomas | 44678 |

| Adams | John | 22456 |

| Adams | John | 24562 |

+----------+-----------+---------+

2.左連接 left [outer] join：

先看左連接的語句與查詢結果

//outer可刪去

mysql> select lastname,firstname,orderNo from persons left join orders on persons.id_p=orders.id_p order by lastname;+----------+-----------+---------+

| lastname | firstname | orderNo |

+----------+-----------+---------+

| Adams | John | 24562 |

| Adams | John | 22456 |

| Bush | George | NULL |

| Carter | Thomas | 77895 |

| Carter | Thomas | 44678 |

+----------+-----------+---------+

通過這個結果我們可以發現lastname為Bush沒有orderNo，而lastname，firstname屬性是來自persons表，orderNo屬性是來自orders表可見左連接的作用時把在left join 左邊persons表格的所有信息都顯示出來，即使是沒有和left join右邊的表 orders中字段相匹配的信息也顯示出來，并且給orderNo的值填為NULL。

如果把left join兩側的 persons和 orders位置互換那么結果顯示的就是left join 左邊表orders表的所有信息，同時把未匹配到的left join右邊表persons中的lastname與firstname值置為NULL，這個結果與未交換join兩個表名位置的右連接的結果一致。

//將left join兩側的 persons和 orders位置互換

mysql> select lastname,firstname,orderNo from orders left join persons on persons.id_p=orders.id_p order by lastname;+----------+-----------+---------+

| lastname | firstname | orderNo |

+----------+-----------+---------+

| NULL | NULL | 34764 |

| Adams | John | 24562 |

| Adams | John | 22456 |

| Carter | Thomas | 44678 |

| Carter | Thomas | 77895 |

+----------+-----------+---------+

3.右連接 right [outer] join:

//outer可刪去

mysql> select lastname,firstname,orderNo from persons right outer join orders on persons.id_p=orders.id_p order by lastname;+----------+-----------+---------+

| lastname | firstname | orderNo |

+----------+-----------+---------+

| NULL | NULL | 34764 |

| Adams | John | 22456 |

| Adams | John | 24562 |

| Carter | Thomas | 77895 |

| Carter | Thomas | 44678 |

+----------+-----------+---------+

同理與左連接，結果顯示的就是right join 左邊表persons表的所有信息，同時把未匹配到的right join右邊表orders中的orderNo值置為NULL。

4.全連接full join(不過mysql/mariadb不支持)

全連接則是把兩個表的信息全部顯示出來，無論兩個表的信息匹配與否，未匹配上的信息都置為NULL。

總結

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