“奇怪”的MYSQL查询语句

[yinghuaxingfu]

select count(*) as nums from online where username is not null
问题：is not null是什么作用，请朋友指教

[Eraserpro]

统计所有username字段不为空的记录的数量吧！
is not null表示条件为username不为空!

[jkit]

is not null 表示非空.
mysql> select count(*) as nums from test where user_name is not null;
+------+
| nums |
+------+
| 5 |
+------+
1 row in set (0.00 sec)