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MySQL中LIKE-inside-SELECT查询的切换

sql-like switch-statement case mysql sql

Sandeepan Nath · 技术社区 · 15 年前

CREATE TABLE IF NOT EXISTS `Tags` (
   `id_tag` int(10) unsigned NOT NULL auto_increment,
   `tag` varchar(255) default NULL,
   PRIMARY KEY  (`id_tag`),
   UNIQUE KEY `tag` (`tag`),
   KEY `id_tag` (`id_tag`),
   KEY `tag_2` (`tag`),
   KEY `tag_3` (`tag`),
   KEY `tag_4` (`tag`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=2937 ;

INSERT INTO `Tags` (`id_tag`, `tag`) VALUES
   (1816, '(class'),
   (2642, 'class\r\n\r\n&acirc;?&not;35'),
   (1906, 'class\r\nif'),
   (1398, 'class'),
   (2436, 'class)'),
   (1973, 'class:\n1.'),
   (2791, 'classes'),
   (1325, 'New'),
   (2185, 'pack'),
   (1905, 'packed'),
   (1389, 'WebClass');

我要获取标记与关键字匹配的所有记录 class 或 pack new

查询1

select id_tag,
case tag 
   when tag LIKE "%class%" then "class" 
   when tag LIKE "%new%" then "new"
   when tag LIKE "%pack%" then "pack"
end as matching_tag 
from Tags 
where tag LIKE "%class%" OR tag LIKE "%new%" OR tag LIKE "%pack%"

查询2

select id_tag,
case tag 
   when "class" then "class" 
   when "new" then "new"
   when "pack" then "pack"
end as matching_tag 
from Tags 
where tag = "class" OR tag = "new" OR tag = "pack"

查询1有什么问题。请帮助。

2 回复 | 直到 10 年前

Unreason 15 年前

Mysql支持case的两个变体,在query2中使用的那个变体灵活性较差,但只支持单个变量上的相等。另一个版本在case和then条件之后不指定变量,条件不必仅相等:

select id_tag,
case  
   when tag LIKE "%class%" then "class" 
   when tag LIKE "%new%" then "new"
   when tag LIKE "%pack%" then "pack"
end as matching_tag 
from Tags 
where tag LIKE "%class%" OR tag LIKE "%new%" OR tag LIKE "%pack%"

documentation 更多细节

编辑: 下面对查询#1返回结果的原因进行更多解释:

case tag
   when tag LIKE "%class%" then "class" 
   when tag LIKE "%new%" then "new"
   when tag LIKE "%pack%" then "pack"
end as matching_tag

期望得到一个文本值,以便在 when ... then 在上述情况下,表达式 tag LIKE "%class%" , tag LIKE "%new%" tag LIKE "%pack%" 都是在实际案例比较之前进行评估的。但是(!),所发生的是,它们变为0或1,当与tag的值比较时,它是0的第一个值,它将匹配任何char(char将被转换为0)-这与第一个查询的结果一致。

select id_tag, tag LIKE "%class%", tag LIKE "%new%", tag = 0, case tag     when tag LIKE "%class%" then "class"     when tag LIKE "%new%" then "new"    when tag LIKE "%pack%" then "pack" end as matching_tag  from Tags  where tag LIKE "%class%" OR tag LIKE "%new%" OR tag LIKE "%pack%";

这就是为什么你会得到意想不到的结果;无声的演员阵容是这里的标准陷阱。

cn007b Dheerendra Kulkarni 10 年前

只是想提醒一下,关于else条款:

case  
   when tag LIKE "%class%" then "class" 
   when tag LIKE "%new%" then "new"
   when tag LIKE "%pack%" then "pack"
   else "no one"
end as matching_tag