mysql null值的特殊处理

实例

尝试以下实例:

root@host# mysql -u root -p password; Enter password:******* mysql> use RUNOOB; Database changed mysql> 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)

转载于:https://www.cnblogs.com/panxuejun/p/6222656.html