mysql>
mysql> drop table topic;
query ok, 0 rows affected (0.00 sec)
<=>null: null不等空
null意味着“没有值”或www.3ppt.com“未知值”，且它被看作与众不同的值。为了测试null，你不能使用算术比较 操作符例如=、<或!=
mysql>
mysql> select name, department, category
-> from topic
-> where category<=>null
-> order by name;
empty set (0.00 sec)
mysql>
mysql> drop table topic;
query ok, 0 rows affected (0.02 sec)
is not null
mysql> select name, department, category
-> from topic
-> where category is not null
-> order by name;
+----------------+------------+----------+
| name | department | category |
+----------------+------------+----------+
| asp.net | popular | country |
| c | classical | dance |
| c sharp | popular | jazz |
| c++ | classical | general |
| java | popular | rock |
| javascript | classical | opera |
| oracle | classical | general |
| perl | classical | vocal |
| php | popular | jazz |
| pl/sql | classical | opera |
| python | popular | blues |
| sql server | classical | general |
| uml | classical | general |
| vb.net | popular | new age |
| vc.net | popular | new age |
| www.java2s.com | classical | dance |
+----------------+------------+----------+
16 rows in set (0.00 sec)
mysql>
mysql> drop table topic;
query ok, 0 rows affected (0.00 sec)
