mysql groupwise max as second where condition - mysql

Question

I have a working query that seems awfully inefficient; I'm wondering if I'm missing a simple way to improve it.
Simple table:
id date master_id
-------------------------
1 2015-02-01 0
2 2015-02-02 0
3 2015-02-03 0
4 2015-02-04 1
5 2015-02-02 1
6 2015-02-17 1
7 2015-02-27 1
8 2015-01-01 1
Objective: Get all rows where the master_id is zero, OR the master_id is not zero and no other rows of the same master_id have an earlier date. Order every result by date.
Current query, using a groupwise minimum subquery to create the second WHERE condition.
SELECT *
FROM `test`
WHERE `master_id` =0
OR `id` IN (
SELECT test.`id`
FROM (
SELECT `master_id`, MIN(`date`) AS mindate
FROM `test`
WHERE `master_id` 0
GROUP BY `master_id`
) AS x
INNER JOIN `test` ON x.`master_id` = test.`master_id`
AND x.mindate= test.`date`
)
ORDER BY `date`
It works, but the EXPLAIN makes it seem inefficient:
id select_type table type possible_keys key key_len ref rows Extra
-------------------------------------------------------------------------------------------------------------
1 PRIMARY test ALL NULL NULL NULL NULL 8 Using where; Using filesort
2 DEPENDENT SUBQUERY derived3 system NULL NULL NULL NULL 1
2 DEPENDENT SUBQUERY test eq_ref PRIMARY PRIMARY 4 func 1 Using where
3 DERIVED test ALL NULL NULL NULL NULL 8 Using where; Using temporary; Using filesort
Can I improve this? Should I break it into two queries, one for ID=0 and one for the groupwise min? Thanks in advance.

Answer 1

Avoiding the inner join can improve the query:
SELECT *
FROM `test`
WHERE `master_id` =0
OR `id` IN (
SELECT t1.id
FROM (SELECT *
FROM test t2
WHERE t2.master_id!=0
ORDER BY t2.date ASC) t1
GROUP BY t1.master_id
)
ORDER BY `date`;

Answer 2

How about this...
SELECT * FROM test WHERE master_id = 0
UNION
SELECT x.*
FROM test x
JOIN (SELECT master_id,MIN(date) min_date FROM test GROUP BY master_id) y
ON y.master_id = x.master_id
AND y.min_date = x.date;