How to group and order dependent results by dates in MySQL - mysql

Question

I have a database of units which have expiry dates. Some of the units are top level units and have sub-assemblies attached. Sub-assemblies also have expiry dates. I'm trying to create a listing that lists the top-level units ordered by their expiration dates and list all relevant sub-assemblies under them.
The ordering should be done in two different ways:
Query which lists top-level units and their sub-assemblies according to expiry date of the parent item.
Query which lists top-level units and their sub-assemblies ordered by the first expiry date of relevant sub-assembly under them.
Here's example of the master table:
ITEM | NAME | UID | INSTALLED_TO_UID | EXPIRY
AAA | Top_level_unit_1 | 1 | | 2018-03-06
BBB | Sub_assy_1 | 75 | 1 | 2019-06-11
AAA | Top_level_unit_2 | 2 | | 2018-08-12
CCC | Sub_assy_2 | 26 | 1 | 2020-02-05
DDD | Sub_assy_3 | 59 | 2 | 2019-11-11
EEE | Sub_assy_4 | 41 | 2 | 2019-10-30
FFF | Sub_assy_5 | 11 | 1 | 2018-04-10
I am running these now in nested queries (second query inside foreach loop), but i am sure there is better and more efficient way of doing this. At least for case 1.
SELECT *
FROM master
WHERE ITEM = AAA
ORDER
BY EXPIRY ASC
And inside result loop for each result:
SELECT *
FROM master
WHERE INSTALLED_TO_UID = (UID from parent query)
The result should be like:
ITEM | NAME | UID | INSTALLED_TO_UID | EXPIRY
AAA | Top_level_unit_1 | 1 | | 2018-03-06
BBB | Sub_assy_1 | 75 | 1 | 2019-06-11
CCC | Sub_assy_2 | 26 | 1 | 2020-02-05
FFF | Sub_assy_5 | 11 | 1 | 2018-04-10
AAA | Top_level_unit_2 | 2 | | 2018-08-12
DDD | Sub_assy_3 | 59 | 2 | 2019-11-11
EEE | Sub_assy_4 | 41 | 2 | 2019-10-30
For case 2 i don't have a clue yet...
Summary: I would like to have only one query to group and order the results instead of relying to another query in result loop. Secondly i would like to figure out a query to order the results by first expiring child item grouped by parent items

Answer 1

You need to use an outer self join to find the parent record (for query 1) or the first-of-children record (for query 2). Then you can use coalesce to decide which value to take (either from the joined table or the main one) for setting the order.
Query 1:
SELECT record.*
FROM master record
LEFT JOIN master parent
ON record.installed_to_uid = parent.uid
AND parent.installed_to_uid is null
ORDER BY COALESCE(parent.expiry, record.expiry),
COALESCE(parent.uid, record.uid),
COALESCE(record.installed_to_uid,-1),
record.expiry
Query 2:
SELECT record.*
FROM master record
LEFT JOIN (SELECT installed_to_uid, MIN(expiry) expiry
FROM master
WHERE installed_to_uid IS NOT NULL
GROUP BY installed_to_uid) first
ON COALESCE(record.installed_to_uid, record.uid) = first.installed_to_uid
ORDER BY COALESCE(first.expiry, record.expiry),
COALESCE(first.installed_to_uid, record.uid),
COALESCE(record.installed_to_uid,-1),
record.expiry
See it run on sqlfiddle

Answer 2

Although there are surely more efficient solutions, why not start with this:
SELECT *
FROM master
WHERE INSTALLED_TO_UID = (
SELECT UID
FROM master
WHERE ITEM = AAA
)
?