mysql的邏輯運算子有四個：與、或、非、異或

我們平時在談論的時候不考慮帶有NULL的情況，今天我們就來考慮一下帶有NULL值，他們的結果是怎麼樣的

AND（&&）運算子

mysql> select 5 AND 6,0 AND 7,0 AND NULL,3 AND NULL,9 AND 2,0 AND 12,0 AND NULL,14 AND NULL;
+---------+---------+------------+------------+---------+----------+------------+-------------+
| 5 AND 6 |
 
 0 AND 7 | 0 AND NULL | 3 AND NULL | 9 AND 2 | 0 AND 12 | 0 AND NULL | 14 AND NULL |
+---------+---------+------------+------------+---------+----------+------------+-------------+
|       1 |       0 |          0 |       NULL |       1 |        0 |          0 |        NULL |
+---------+---------+------------+------------+---------+----------+------------+-------------+
 

1 row in set (0.00 sec)

OR（||）運算子

mysql> select 5 OR 6,0 OR 7,0 OR 0,3 OR NULL,9 || 2,0 || 12,0 || NULL,14 || NULL;
+--------+--------+--------+-----------+--------+---------+-----------+------------+
| 5 OR 6 | 0 OR 7 | 0 OR 0 | 3 OR NULL | 9 || 2 | 0 || 12 | 0 || NULL | 14 || NULL |
+--------+--------+--------+-----------+--------+---------+-----------+------------+
 

|      1 |      1 |      0 |         1 |      1 |       1 |      NULL |          1 |
+--------+--------+--------+-----------+--------+---------+-----------+------------+
1 row in set (0.01 sec)

NOT( ! )運算子

mysql> select  NOT 5,NOT 0,NOT NULL, NOT 3,NOT 0;
+-------+-------+----------+-------+-------+
| NOT 5 | NOT 0 | NOT NULL | NOT 3 | NOT 0 |
+-------+-------+----------+-------+-------+
|     0 |     1 |     NULL |     0 |     1 |
+-------+-------+----------+-------+-------+
1 row in set (0.00 sec)

總結：通過上面的分析我發現：NULL值是處於0和1之間的某個值，他也表示一個值，只不過這個值是NULL值，而不是0。在進行計算的時候，1與NULL則結果為NULL。而0與NULL則結果為0。1或NULL則結果為1，0或NULL則結果為NULL；可見NULL值是介於0和1之間的值。另外非NULL既不是1也不是0，還是NULL。