理论小知识：在命令提示符中使用 NULL 值

以下实例中假设数据库 RUNOOB 中的表 runoob_test_tbl 含有两列 runoob_author 和 runoob_count, runoob_count 中设置插入NULL值。
实例
尝试以下实例:
创建数据表 runoob_test_tbl
root@host# mysql -u root -p password; Enter password:******* mysql> use RUNOOB; Database changed mysql> create table runoob_test_tbl -> ( -> runoob_author varchar(40) NOT NULL, -> runoob_count INT -> ); Query OK, 0 rows affected (0.05 sec) mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values (‘RUNOOB’, 20); mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values (‘programb’, NULL); mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values (‘Google’, NULL); mysql> INSERT INTO runoob_test_tbl (runoob_author, runoob_count) values (‘FK’, 20); mysql> SELECT * from runoob_test_tbl; ±--------------±-------------+ | runoob_author | runoob_count | ±--------------±-------------+ | RUNOOB | 20 | | programb| NULL | | Google | NULL | | FK | 20 | ±--------------±-------------+ 4 rows in set (0.01 sec)
以下实例中你可以看到 = 和 != 运算符是不起作用的：
mysql> SELECT * FROM runoob_test_tbl WHERE runoob_count = NULL; Empty set (0.00 sec) mysql> SELECT * FROM runoob_test_tbl WHERE runoob_count != NULL; Empty set (0.01 sec)
查找数据表中 runoob_test_tbl 列是否为 NULL，必须使用 IS NULL 和 IS NOT NULL，如下实例：
mysql> SELECT * FROM runoob_test_tbl WHERE runoob_count IS NULL; ±--------------±-------------+ | runoob_author | runoob_count | ±--------------±-------------+ | programb| NULL | | Google | NULL | ±--------------±-------------+ 2 rows in set (0.01 sec) mysql> SELECT * from runoob_test_tbl WHERE runoob_count IS NOT NULL; ±--------------±-------------+ | runoob_author | runoob_count | ±--------------±-------------+ | RUNOOB | 20 | | FK | 20 | ±--------------±-------------+ 2 rows in set (0.01 sec)