MySql - Get field from table 2, If not then from table 1 - mysql

Question

I have two table complain and repair. I want to get ass_to_per [Latest one] from repair or from complain if the id of complain is not present in repair.
Explanation:
I want to get the ass_to_per from the complain table, and also from the repair table. But there is a relation between them, in repair there is a field called com_id which is a foreign key. So i want to get the ass_to_per from complain and also check the repair for the foreign key, if any then check the ass_to_per of repair. If any then get it as result.
I have sqlfiddle, For online testing: sqlfiddle.com
The table and data given below.
Complain
CREATE TABLE `complain` (
`id` int(11) NOT NULL,
`ass_to_per` varchar(50) NOT NULL
);
INSERT INTO `complain` (`id`, `ass_to_per`) VALUES
(1, 'frayne'),
(2, 'murad'),
(4, 'frayne'),
(5, 'murad'),
(6, 'frayne'),
(7, 'frayne');
Repair
CREATE TABLE `repair` (
`id` int(11) NOT NULL,
`com_id` int(11) NOT NULL,
`ass_to_per` varchar(50) NOT NULL
);
INSERT INTO `repair` (`id`, `com_id`, `ass_to_per`) VALUES
(1, 1, 'frayne'),
(2, 1, 'murad'),
(3, 4, 'frayne'),
(4, 6, 'murad'),
(5, 2, 'murad'),
(6, 5, 'frayne');
My Query:
SELECT * FROM `complain`
WHERE `id` IN (SELECT DISTINCT(`com_id`) FROM `repair` WHERE `ass_to_per` = 'frayne') OR `ass_to_per`='frayne'
Query Result
id | ass_to_per
--------------
1 | frayne
4 | frayne
5 | murad
6 | frayne
7 | frayne
Analysis
id | ass_to_per[complain] | ass_to_per[repair]
--------------
1 | murad | frayne
2 | murad | murad
4 | frayne | frayne //need this one
5 | murad | frayne //need this one
6 | frayne | murad
7 | frayne | //need this one
Expected result:
id | ass_to_per
--------------
4 | frayne //ass_to_per from repair
5 | frayne //ass_to_per from repair
7 | frayne //ass_to_per from complain

Answer 1

COALESCE() will output the first non-null parameter it finds, so using a join you can probably get ass_to_per from complain or repair depending on which one exists:
SELECT
complain.id,
COALESCE(repair3.ass_to_per, complain.ass_to_per) as ass_to_per
FROM complain
LEFT JOIN
(SELECT max(id) as maxid, com_id FROM repair GROUP BY com_id) as repair2
ON complain.id = repair2.com_id
LEFT JOIN repair as repair3
ON repair2.maxid = repair3.id
GROUP BY complain.id
If you want to further filter (like in your example on 'frayne') on computed ass_to_per, just embed this select as a subquery:
SELECT *
FROM (
SELECT
complain.id,
COALESCE(repair3.ass_to_per, complain.ass_to_per) as ass_to_per
FROM complain
LEFT JOIN (SELECT max(id) as maxid, com_id FROM repair GROUP BY com_id) AS repair2
ON complain.id = repair2.com_id
LEFT JOIN repair as repair3
ON repair2.maxid = repair3.id
GROUP BY complain.id
) AS mydata
WHERE mydata.ass_to_per = 'frayne'
ORDER BY mydata.id;
Fiddle here : http://sqlfiddle.com/#!9/33433/49

Answer 2

SELECT c.id
FROM complain c
LEFT
JOIN
( SELECT x.*
FROM repair x
JOIN
( SELECT com_id,MAX(id) id FROM repair GROUP BY com_id ) y
ON y.com_id = x.com_id
AND y.id = x.id
) r
ON r.com_id = c.id
WHERE COALESCE(r.ass_to_per,c.ass_to_per) = 'frayne';