union - How do I select some rows and then others in MySQL?

Question

I have this query, it works but I'm not sure if it's the best approach and I don't get what I want.

I need to select the query contained in the "IN" clause first, then union with others. Entire row returned must be 40.

SELECT * 
FROM (
    SELECT * FROM tbl_x a WHERE id IN(11,20,30) 
    UNION ALL 
    SELECT * FROM tbl_x b WHERE exam_group='jpx' AND subject='chemistry'
) ab 
GROUP BY id LIMIT 40

Answer 1

The next query should to return same data in simple way:

SELECT * 
FROM tbl_x 
WHERE 
    id IN (11,20,30)
    OR (exam_group='jpx' AND subject='chemistry')
ORDER BY id IN (11,20,30) DESC, id
LIMIT 40;