在MySQL中的SELECT查询中使用LIKE进行SWITCH

Question

我有这个标签表

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\nâ?¬35'),
   (1906, 'class\r\nif'),
   (1398, 'class'),
   (2436, 'class)'),
   (1973, 'class:\n1.'),
   (2791, 'classes'),
   (1325, 'New'),
   (2185, 'pack'),
   (1905, 'packed'),
   (1389, 'WebClass');

我想取其中标签匹配关键字的所有记录class或pack或new与这表明该3个关键字的实际与标记字段匹配的另一场一起.

以下查询未提供正确的结果查询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%"

我必须在案例中使用类似的东西.否则完全匹配工作.以下查询有效: -

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"

查询有什么问题1.请帮忙.

Answer 1

Mysql支持两种大小写的变体,您在查询2中使用的变体不太灵活,但仅支持单个变量上的相等性.另一个版本在case之后没有指定变量,那么条件不必仅仅是相等的:

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%"

有关详细信息,请参阅文档

编辑:这里有一个更多的解释为什么你的查询#1返回它返回的内容:

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

期望获得用于比较的文字值.when ... then 在上面的情况下,表达式tag LIKE "%class%",tag LIKE "%new%"并且tag LIKE "%pack%"在实际案例比较之前都进行了评估.然而(!),会发生的是它们变为0或1,并且当与tag的值相比时,它是第一个值0将匹配任何char(char将被转换为0) - 这与结果一致你的第一个查询.

这是一个查询,显示相关表达式的逻辑值:

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%";

这就是你得到意想不到的结果的原因; 沉默的CAST是一个标准的陷阱.

Answer 2

只是想提醒一下,关于else子句:

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