MYSQL教程mysql中is null语句的用法分享

发布时间：2020-12-12 01:03:54 所属栏目：MySql教程来源：网络整理

导读：《MYSQL教程mysql中is null语句的用法分享》要点：本文介绍了MYSQL教程mysql中is null语句的用法分享，希望对您有用。如果有疑问，可以联系我们。 mysql数据库中is null语句的用法 MYSQL入门注意在mysql中,0或 null意味着假而其它值意味着真.布尔运算的默

《MYSQL教程mysql中is null语句的用法分享》要点：
本文介绍了MYSQL教程mysql中is null语句的用法分享，希望对您有用。如果有疑问，可以联系我们。

mysql数据库中is null语句的用法MYSQL入门

注意在mysql中,0或 null意味着假而其它值意味着真.布尔运算的默认真值是1.MYSQL入门

对null的特殊处理即是在前面的章节中,为了决定哪个动物不再是活着的,使用death is not null而不使用death != null的原因.MYSQL入门

在group by中,两个null值视为相同.MYSQL入门

执行order by时,如果运行 order by ... asc,则null值出现在最前面,若运行order by ... desc,则null值出现在最后面.MYSQL入门

null操作的常见错误是不能在定义为not null的列内插入0或空字符串,但事实并非如此.在null表示"没有数值"的地方有数值MYSQL入门

.使用is [not] null则可以很容易地进行测试MYSQL入门

is null or = nullMYSQL入门

mysql>
mysql> create table topic(
??? ->??? topicid???? smallint not null auto_increment primary key,
??? ->??? name??????? varchar(50) not null,
??? ->??? instock???? smallint unsigned not null,
??? ->??? onorder???? smallint unsigned not null,
??? ->??? reserved??? smallint unsigned not null,
??? ->??? department? enum('classical','popular') not null,
??? ->??? category??? varchar(20) not null,
??? ->??? rowupdate?? timestamp not null
??? -> );
query ok,0 rows affected (0.02 sec)MYSQL入门

mysql>
mysql>
mysql> insert into topic (name,????????? instock,onorder,reserved,department,?? category) values
??? ->?????????????????? ('java',????????? 10,????? 5,?????? 3,??????? 'popular',??? 'rock'),
??? ->?????????????????? ('css',??? 10,??????? 'classical',? 'opera'),
??? ->?????????????????? ('c sharp',?????? 17,????? 4,?????? 1,??? 'jazz'),
??? ->?????????????????? ('c',???????????? 9,?????? 4,?????? 2,? 'dance'),
??? ->?????????????????? ('c++',?????????? 24,????? 2,?????? 5,? 'general'),
??? ->?????????????????? ('perl',????????? 16,????? 6,?????? 8,? 'vocal'),
??? ->?????????????????? ('python',??????? 2,?????? 25,??? 'blues'),
??? ->?????????????????? ('php',?????????? 32,????? 3,?????? 10,?????? 'popular',
??? ->?????????????????? ('asp.net',?????? 12,????? 15,????? 13,??? 'country'),
??? ->?????????????????? ('vb.net',??????? 5,?????? 20,????? 10,??? 'new age'),
??? ->?????????????????? ('vc.net',??????? 24,????? 11,????? 14,
??? ->?????????????????? ('uml',?????????? 42,????? 17,?????? 'classical',
??? ->?????????????????? ('www.java2s.com',25,????? 44,????? 28,
??? ->?????????????????? ('oracle',??????? 32,????? 12,
??? ->?????????????????? ('pl/sql',??????? 20,
??? ->?????????????????? ('sql server',??? 23,????? 8,? 'general');
query ok,16 rows affected (0.00 sec)
records: 16? duplicates: 0? warnings: 0MYSQL入门

mysql>
mysql> select * from topic;
+---------+----------------+---------+---------+----------+------------+----------+---------------------+
| topicid | name?????????? | instock | onorder | reserved | department | category | rowupdate?????????? |
+---------+----------------+---------+---------+----------+------------+----------+---------------------+
|?????? 1 | java?????????? |????? 10 |?????? 5 |??????? 3 | popular??? | rock???? | 2007-07-23 19:09:45 |
|?????? 2 | javascript???? |????? 10 |?????? 5 |??????? 3 | classical? | opera??? | 2007-07-23 19:09:45 |
|?????? 3 | c sharp??????? |????? 17 |?????? 4 |??????? 1 | popular??? | jazz???? | 2007-07-23 19:09:45 |
|?????? 4 | c????????????? |?????? 9 |?????? 4 |??????? 2 | classical? | dance??? | 2007-07-23 19:09:45 |
|?????? 5 | c++??????????? |????? 24 |?????? 2 |??????? 5 | classical? | general? | 2007-07-23 19:09:45 |
|?????? 6 | perl?????????? |????? 16 |?????? 6 |??????? 8 | classical? | vocal??? | 2007-07-23 19:09:45 |
|?????? 7 | python???????? |?????? 2 |????? 25 |??????? 6 | popular??? | blues??? | 2007-07-23 19:09:45 |
|?????? 8 | php??????????? |????? 32 |?????? 3 |?????? 10 | popular??? | jazz???? | 2007-07-23 19:09:45 |
|?????? 9 | asp.net??????? |????? 12 |????? 15 |?????? 13 | popular??? | country? | 2007-07-23 19:09:45 |
|????? 10 | vb.net???????? |?????? 5 |????? 20 |?????? 10 | popular??? | new age? | 2007-07-23 19:09:45 |
|????? 11 | vc.net???????? |????? 24 |????? 11 |?????? 14 | popular??? | new age? | 2007-07-23 19:09:45 |
|????? 12 | uml??????????? |????? 42 |????? 17 |?????? 17 | classical? | general? | 2007-07-23 19:09:45 |
|????? 13 | www.java2s.com |????? 25 |????? 44 |?????? 28 | classical? | dance??? | 2007-07-23 19:09:45 |
|????? 14 | oracle???????? |????? 32 |????? 15 |?????? 12 | classical? | general? | 2007-07-23 19:09:45 |
|????? 15 | pl/sql???????? |????? 20 |????? 10 |??????? 5 | classical? | opera??? | 2007-07-23 19:09:45 |
|????? 16 | sql server???? |????? 23 |????? 12 |??????? 8 | classical? | general? | 2007-07-23 19:09:45 |
+---------+----------------+---------+---------+----------+------------+----------+---------------------+
16 rows in set (0.00 sec)MYSQL入门

mysql>
mysql>
mysql> select name,category
??? -> from topic
??? -> where category is null
??? -> order by name;
empty set (0.00 sec)MYSQL入门

mysql>
mysql>
mysql>
mysql> select name,category
??? -> from topic
??? -> where category = null
??? -> order by name;
empty set (0.00 sec)MYSQL入门

mysql>
mysql>
mysql> drop table topic;
query ok,0 rows affected (0.00 sec)MYSQL入门

?<=>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,category
??? -> from topic
??? -> where category<=>null
??? -> order by name;
empty set (0.00 sec)MYSQL入门

mysql>
mysql> drop table topic;
query ok,0 rows affected (0.02 sec)MYSQL入门

is not nullMYSQL入门

mysql>
mysql> drop table topic;
query ok,0 rows affected (0.00 sec)MYSQL入门

（编辑：李大同）

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