舉例數據庫：
mysql> SELECT * FROM PET;
+----------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+--------+---------+------+------------+------------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Fang | Benny | dog | m | 1990-08-27 | NULL |
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
| Bowser | Diane | dog | m | 1979-08-31 | 1995-07-29 |
+----------+--------+---------+------+------------+------------+
1. NULL值不能用於比較，因爲任何值和NULL值比較永遠是NULL
mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL, NULL = NULL;
+----------+-----------+----------+----------+-------------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL | NULL = NULL |
+----------+-----------+----------+----------+-------------+
| NULL | NULL | NULL | NULL | NULL |
+----------+-----------+----------+----------+-------------+
所以，尋找NULL值不能使用比較運算符，應該使用IS [NOT] NULL：
mysql> SELECT * FROM PET WHERE DEATH = NULL;
Empty set (0.01 sec)
mysql> SELECT * FROM PET WHERE DEATH IS NULL;
+----------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+----------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Fang | Benny | dog | m | 1990-08-27 | NULL |
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
+----------+--------+---------+------+------------+-------+
2. NULL值在ORDER BY或者GROUP BY子句中按照相同對待
mysql> SELECT * FROM PET ORDER BY DEATH;
+----------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+--------+---------+------+------------+------------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Fang | Benny | dog | m | 1990-08-27 | NULL |
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
| Bowser | Diane | dog | m | 1979-08-31 | 1995-07-29 |
+----------+--------+---------+------+------------+------------+
3. 0和' '不是NULL，完全可能插入到NOT NULL的列裏
將PET表中的name字段改爲NOT NULL，然後新加記錄：
mysql> ALTER TABLE PET MODIFY COLUMN name VARCHAR(20) NOT NULL;
Query OK, 0 rows affected (0.10 sec)
mysql> DESCRIBE PET;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | NO | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
mysql> INSERT INTO PET VALUES(NULL, 'Matthew', 'cat', 'f', '2010-01-04', NULL);
ERROR 1048 (23000): Column 'name' cannot be null
mysql> INSERT INTO PET VALUES('', 'Matthew', 'cat', 'f', '2010-01-04', NULL);
Query OK, 1 row affected (0.01 sec)
mysql> INSERT INTO PET VALUES(0, 'Matthew', 'cat', 'f', '2010-01-04', NULL);
Query OK, 1 row affected (0.01 sec)
4. 在MySQL中，0和NULL意味着FALSE，其他值意味着TRUE
mysql> SELECT * FROM PET WHERE 1 <> 1;
Empty set (0.00 sec)
mysql> SELECT * FROM PET WHERE 1 <> NULL;
Empty set (0.00 sec)
5. 空值不佔用空間，但NULL值佔用空間
舉個例子：假設你有一個杯子，空值代表杯子是真空的，NULL值代表杯子中裝滿了空氣
