Lets say you have a table with field called "tags" that consists from tags separated by commas and you want to check whether it includes a required tag. Here the simplest ways of doing that.

Search in Tags Set in MySQL

-- source: http://www.apphp.com/index.php?snippet=mysql-search-in-tags-set#null
-- 1st way
SELECT *
FROM TABLE
WHERE FIND_IN_SET(" '.$tag.'", CONCAT(" ", TABLE.tags))
LIMIT 1;
 
-- 2st way 
SELECT *
FROM TABLE
WHERE '.$tag.' IN(TABLE.tags)
LIMIT 1;