判断字段值是否为null，该使用 '=null' 还是 'is

结论: 一定要使用 'is null' 来判断一个字段的值是否为null

任何和NULL值做比较的表达式的值都为NULL，就是这样：

mysql> SELECT 1 = NULL;
+----------+
| 1 = NULL |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

mysql> SELECT 1 != NULL;
+-----------+
| 1 != NULL |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

mysql> SELECT NULL = NULL;
+-------------+
| NULL = NULL |
+-------------+
|        NULL |
+-------------+
1 row in set (0.00 sec)

mysql> SELECT NULL != NULL;
+--------------+
| NULL != NULL |
+--------------+
|         NULL |
+--------------+
1 row in set (0.00 sec)

而 'is null' 语句能够返回预期结果：

mysql> select null is null;
+--------------+
| null is null |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)