以下实例中假设数据库 TUTORIALS 中的表 tcount_tbl 含有两列 tutorial_author 和 tutorial_count, tutorial_count 中设置插入NULL值。

实例

尝试以下实例:

root@host# mysql -u root -p password;
Enter password:*******
mysql> use TUTORIALS;
Database changed
mysql> create table tcount_tbl
    -> (
    -> tutorial_author varchar(40) NOT NULL,
    -> tutorial_count  INT
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> INSERT INTO tcount_tbl
    -> (tutorial_author, tutorial_count) values ('mahran', 20);
mysql> INSERT INTO tcount_tbl
    -> (tutorial_author, tutorial_count) values ('mahnaz', NULL);
mysql> INSERT INTO tcount_tbl
    -> (tutorial_author, tutorial_count) values ('Jen', NULL);
mysql> INSERT INTO tcount_tbl
    -> (tutorial_author, tutorial_count) values ('Gill', 20);

mysql> SELECT * from tcount_tbl;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran          |             20 |
| mahnaz          |           NULL |
| Jen             |           NULL |
| Gill            |             20 |
+-----------------+----------------+
4 rows in set (0.00 sec)

mysql>

以下实例中你可以看到 = 和 != 运算符是不起作用的：

mysql> SELECT * FROM tcount_tbl WHERE tutorial_count = NULL;
Empty set (0.00 sec)
mysql> SELECT * FROM tcount_tbl WHERE tutorial_count != NULL;
Empty set (0.01 sec)

查找数据表中 tutorial_count 列是否为 NULL，必须使用IS NULL和IS NOT NULL，如下实例：

mysql> SELECT * FROM tcount_tbl 
    -> WHERE tutorial_count IS NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahnaz          |           NULL |
| Jen             |           NULL |
+-----------------+----------------+
2 rows in set (0.00 sec)
mysql> SELECT * from tcount_tbl 
    -> WHERE tutorial_count IS NOT NULL;
+-----------------+----------------+
| tutorial_author | tutorial_count |
+-----------------+----------------+
| mahran          |             20 |
| Gill            |             20 |
+-----------------+----------------+
2 rows in set (0.00 sec)