实例

尝试以下实例:

root@host# mysql -u root -p password;Enter password:*******mysql> use RUNOOB;Database changedmysql> create table tcount_tbl    -> (    -> runoob_author varchar(40) NOT NULL,    -> runoob_count  INT    -> );Query OK, 0 rows affected (0.05 sec)mysql> INSERT INTO tcount_tbl    -> (runoob_author, runoob_count) values (‘mahran‘, 20);mysql> INSERT INTO tcount_tbl    -> (runoob_author, runoob_count) values (‘mahnaz‘, NULL);mysql> INSERT INTO tcount_tbl    -> (runoob_author, runoob_count) values (‘Jen‘, NULL);mysql> INSERT INTO tcount_tbl    -> (runoob_author, runoob_count) values (‘Gill‘, 20);mysql> SELECT * from tcount_tbl;+-----------------+----------------+| runoob_author | runoob_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 runoob_count = NULL;Empty set (0.00 sec)mysql> SELECT * FROM tcount_tbl WHERE runoob_count != NULL;Empty set (0.01 sec)

实例

查找数据表中 runoob_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 runoob_count IS NULL;+-----------------+----------------+| runoob_author | runoob_count |+-----------------+----------------+| mahnaz          |           NULL || Jen             |           NULL |+-----------------+----------------+2 rows in set (0.00 sec)mysql> SELECT * from tcount_tbl     -> WHERE runoob_count IS NOT NULL;+-----------------+----------------+| runoob_author | runoob_count |+-----------------+----------------+| mahran          |             20 || Gill            |             20 |+-----------------+----------------+2 rows in set (0.00 sec)

mysql null值的特殊处理