Question

I have this Tags table

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');

I want to fetch all records where tag matches keywords class or pack or new, along with another field which indicates which of the 3 keywords actually matched with the tag field.

The following query does not give correct results
Query 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%"

I have to use the like inside the case. Otherwise complete matching works. The following query works:-

Query 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"

What is wrong with the query 1. Please help.

Answer 1

Mysql supports two variants of case, the one you use in query 2 is less flexible but supports only equality on a single variable. The other version specifies no variable after case and then conditions need not be only equality:

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%"

See documentation for further details

EDIT:
Here’s a bit more explanation on why your query #1 returned what it returned:

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

expects to get a literal value for comparison between when ... then
In the above case the expressions tag LIKE "%class%", tag LIKE "%new%" and tag LIKE "%pack%" are all evaluated before the actual case comparison.
However (!), what happens is that they become either 0 or 1 and when compared to the value of tag it is the first value of 0 that will match any char (char will get cast to 0) – this is consistent with the results of your first query.

Here’s a query that shows the logical values for the relevant expressions:

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%";

That’s why you get unexpected results; the silent CAST is a standard pitfall here.