ORDER BY (case when `cat` = 'mgr' then 1 
    when `cat` = 'dev' then 2 
    else 3 end)

This structure help me in Wordpress. For example to select custom posts first theh other: poster and project  is custom
 SELECT `post_type` FROM `wp_posts` ORDER BY (case when `post_type` = 'poster' then 1 when `post_type` = 'project' then 2 else 3 end)