定義：

LEFT JOIN 關鍵字會從左表 (table_name1) 那裡返回所有的行，即使在右表 (table_name2) 中沒有匹配的行。例如如下查詢：

SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

此查詢中on的條件只有一個，因此不存在特殊注意之處。但是當我們on條件如果存在多個時候會出現一些與我們預期不符的查詢結果。下面以具體例子說明。

基礎查詢資料表

使用者表：

mysql> select * from tab_user;
+---------+------+--------+----------+
| name    | age  | sex    | addr     |
+---------+------+--------+----------+
| daxin   |   18 | male   | beijing  |
| mali    |   28 | female | shandong |
| wangsan |   34 | male   | beijing  |
| lisi    |   45 | male   | liaoning |
| liwu    |   58 | female | beijing  |
| maoliu  |   43 | male   | anhui    |
| zhouba  |   62 | female | beijing  |
+---------+------+--------+----------+
7 rows in set (0.00 sec)
 

訂單表：

mysql> select * from tab_order;
+-------+-----------+
| name  | gname     |
+-------+-----------+
| daxin | Smartisan |
| mali  | iPhone    |
| liwu  | Mac       |
| lisi  | xiaomi    |
| maliu | nike      |
+-------+-----------+
5 rows in set (0.00 sec)

查詢語句1：

mysql> select * from tab_user u left join tab_order o on u.name=o.name and u.name='lisi';
+---------+------+--------+----------+------+--------+
| name    | age  | sex    | addr     | name | gname  |
+---------+------+--------+----------+------+--------+
| daxin   |   18 | male   | beijing  | NULL | NULL   |
| mali    |   28 | female | shandong | NULL | NULL   |
| wangsan |   34 | male   | beijing  | NULL | NULL   |
| lisi    |   45 | male   | liaoning | lisi | xiaomi |
| liwu    |   58 | female | beijing  | NULL | NULL   |
| maoliu  |   43 | male   | anhui    | NULL | NULL   |
| zhouba  |   62 | female | beijing  | NULL | NULL   |
+---------+------+--------+----------+------+--------+
7 rows in set (0.00 sec)
 

咋一看是不是很蒙圈，為什麼已經限制了u.name='lisi'卻查詢結果還有lisi呢？如果換用where約束，查詢語句2.

查詢語句2：

mysql> select * from tab_user u left join tab_order o on u.name=o.name where u.name='lisi';
+------+------+------+----------+------+--------+
| name | age  | sex  | addr     | name | gname  |
+------+------+------+----------+------+--------+
| lisi |   45 | male | liaoning | lisi | xiaomi |
+------+------+------+----------+------+--------+
1 row in set (0.00 sec)
 

這次確實只有lisi了。那為什麼查詢語句1會與預期不符？回顧一下left join的定義，左邊表會返回所有行，所以left join如果對左邊表進行約束的話是不會生效的；但是，對left join的右邊表新增條件的話是生效的！反之，right join 同理！