//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 * from orders;
+------+----------+-----------+----------------+----------+
| 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 (0.05 sec)

+------+---------+------+
| 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，並且給orderNo的值填為NULL。

　　如果把left join兩側的 persons和 orders位置互換那麽結果顯示的就是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。

mysql/mariadb學習記錄——連接查詢（JOIN）