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NOT IN 的作用和 IN 恰好相反，NOT IN 用來(lái)判斷表達(dá)式的值是否不存在于給出的列表中；如果不是，返回值為 1，否則返回值為 0。

IN 和 NOT IN 的語(yǔ)法格式如下：

expr IN ( value1, value2, value3 ... valueN )
expr NOT IN ( value1, value2, value3 ... valueN )

expr 表示要判斷的表達(dá)式，value1, value2, value3 ... valueN 表示列表中的值。MySQL 會(huì)將 expr 的值和列表中的值逐一對(duì)比。

【實(shí)例1】在 SQL 語(yǔ)句中使用 IN 和 NOT IN 運(yùn)算符：

mysql> SELECT 2 IN (1,3,5,'thks'),'thks' IN (1,3,5, 'thks');
+---------------------+---------------------------+
| 2 IN (1,3,5,'thks') | 'thks' IN (1,3,5, 'thks') |
+---------------------+---------------------------+
|                   0 |                         1 |
+---------------------+---------------------------+
1 row in set, 2 warnings (0.00 sec)

mysql> SELECT 2 NOT IN (1,3,5,'thks'),'thks' NOT IN (1,3,5, 'thks');
+-------------------------+-------------------------------+
| 2 NOT IN (1,3,5,'thks') | 'thks' NOT IN (1,3,5, 'thks') |
+-------------------------+-------------------------------+
|                       1 |                             0 |
+-------------------------+-------------------------------+
1 row in set, 2 warnings (0.00 sec)

由結(jié)果可以看到，IN 和 NOT IN 的返回值正好相反。

對(duì)空值 NULL 的處理

當(dāng) IN 運(yùn)算符的兩側(cè)有一個(gè)為空值 NULL 時(shí)，如果找不到匹配項(xiàng)，則返回值為 NULL；如果找到了匹配項(xiàng)，則返回值為 1。

請(qǐng)看下面的 SQL 語(yǔ)句如下：

mysql> SELECT NULL IN (1,3,5,'thks'),10 IN (1,3,NULL,'thks');
+------------------------+-------------------------+
| NULL IN (1,3,5,'thks') | 10 IN (1,3,NULL,'thks') |
+------------------------+-------------------------+
|                   NULL |                    NULL |
+------------------------+-------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>  SELECT NULL IN (1,3,5,'thks'),10 IN (1,10,NULL,'thks');
+------------------------+--------------------------+
| NULL IN (1,3,5,'thks') | 10 IN (1,10,NULL,'thks') |
+------------------------+--------------------------+
|                   NULL |                        1 |
+------------------------+--------------------------+
1 row in set (0.00 sec)

NOT IN 恰好相反，當(dāng) NOT IN 運(yùn)算符的兩側(cè)有一個(gè)為空值 NULL 時(shí)，如果找不到匹配項(xiàng)，則返回值為 NULL；如果找到了匹配項(xiàng)，則返回值為 0。

請(qǐng)看下面的 SQL 語(yǔ)句如下：

mysql>  SELECT NULL NOT IN (1,3,5,'thks'),10 NOT IN (1,0,NULL,'thks');
+----------------------------+-----------------------------+
| NULL NOT IN (1,3,5,'thks') | 10 NOT IN (1,0,NULL,'thks') |
+----------------------------+-----------------------------+
|                       NULL |                        NULL |
+----------------------------+-----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>  SELECT NULL NOT IN (1,3,5,'thks'),10 NOT IN (1,10,NULL,'thks');
+----------------------------+------------------------------+
| NULL NOT IN (1,3,5,'thks') | 10 NOT IN (1,10,NULL,'thks') |
+----------------------------+------------------------------+
|                       NULL |                            0 |
+----------------------------+------------------------------+
1 row in set (0.00 sec)