結(jié)論: 一定要使用 'is null' 來判斷一個(gè)字段的值是否為null
任何和NULL值做比較的表達(dá)式的值都為NULL钱反，就是這樣：
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' 語句能夠返回預(yù)期結(jié)果：
mysql> select null is null;
+--------------+
| null is null |
+--------------+
| 1 |
+--------------+
1 row in set (0.00 sec)
