LIKE '%3%' will incorrectly catch 13.
LIKE '%,3,%' will fail on '3,4,5,6,7'
etc.
This is a common discussion:
http://forums.mysql.com/read.php?10,274102 (Setting up some new tables)
http://forums.mysql.com/read.php?10,259969 (parse by delimiter and search a data field?)
http://forums.mysql.com/read.php?10,258430 (Iterating through SQL table)
http://forums.mysql.com/read.php?20,244989 (Split Values by comma and Check in another Table?)
http://forums.mysql.com/read.php?10,249227 (Column for mutiple and not-predefined values)
http://forums.mysql.com/read.php?10,277624 (MYSQL LIKE and REGEXP style query help needed)
http://forums.mysql.com/read.php?10,271995 (help in select ... where ...like...)
http://forums.mysql.com/read.php?10,206857,255573 (search comma separated db column)
In those you will find 3 things:
* "Don't design your table that way!"
* FIND_IN_SET() (slow)
* a REGEXP (RLIKE) that does work (slow)