mysql数据库中is null语句的用法 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。 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。 在group by中，两个null值视为相同。 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。 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 。使用is [not] null则可以很容易地进行测试 is null or = null 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> mysql> mysql> insert into topic (name,          instock, onorder, reserved, department,   category) values     ->                   ('java',          10,      5,       3,        'popular',    'rock'),     ->                   ('css',    10,      5,       3,        'classical',  'opera'),     ->                   ('c sharp',       17,      4,       1,        'popular',    'jazz'),     ->                   ('c',             9,       4,       2,        'classical',  'dance'),     ->                   ('c++',           24,      2,       5,        'classical',  'general'),     ->                   ('perl',          16,      6,       8,        'classical',  'vocal'),     ->                   ('python',        2,       25,      6,        'popular',    'blues'),     ->                   ('php',           32,      3,       10,       'popular',    'jazz'),     ->                   ('asp.net',       12,      15,      13,       'popular',    'country'),     ->                   ('vb.net',        5,       20,      10,       'popular',    'new age'),     ->                   ('vc.net',        24,      11,      14,       'popular',    'new age'),     ->                   ('uml',           42,      17,      17,       'classical',  'general'),     ->                   ('www.java2s.com',25,      44,      28,       'classical',  'dance'),     ->                   ('oracle',        32,      15,      12,       'classical',  'general'),     ->                   ('pl/sql',        20,      10,      5,        'classical',  'opera'),     ->                   ('sql server',    23,      12,      8,        'classical',  'general'); query ok, 16 rows affected (0.00 sec) records: 16  duplicates: 0  warnings: 0 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> mysql> mysql> select name, department, category     -> from topic     -> where category is null     -> order by name; empty set (0.00 sec) mysql> mysql> mysql> mysql> select name, department, category     -> from topic     -> where category = null     -> order by name; empty set (0.00 sec) mysql> 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)