SQL Joins for Multiple Fields with Null Values - sql

Question

I have a table of maintenance requirements and associated monthly frequency it is to be performed
maint
+----------+------+
| maint_id | freq |
+----------+------+
| 1 | 6 |
| 2 | 12 |
| 3 | 24 |
| 4 | 3 |
+----------+------+
I also have a table of equipment with data on its manufacturer, model, device type and building.
equip
+----------+--------+--------+--------+---------+
| equip_id | mfg_id | mod_id | dev_id | bldg_id |
+----------+--------+--------+--------+---------+
| 1 | 1 | 1 | 3 | 1 |
| 2 | 1 | 2 | 3 | 1 |
| 3 | 2 | 3 | 1 | 2 |
| 4 | 2 | 3 | 1 | 3 |
+----------+--------+--------+--------+---------+
I am trying to match each maintenance requirement with its associated equipment. Each requirement applies to a specific manufacturer, model, device, facility or any combination of these in its scope of application.
I have created a table to manage these relationships like this:
maint_equip
+----------------+----------+--------+--------+--------+---------+
| maint_equip_id | maint_id | mfg_id | mod_id | dev_id | bldg_id |
+----------------+----------+--------+--------+--------+---------+
| 1 | 1 | NULL | NULL | 1 | NULL |
| 2 | 2 | 2 | NULL | NULL | 2 |
| 3 | 3 | NULL | NULL | NULL | 1 |
| 4 | 3 | NULL | NULL | NULL | 3 |
| 5 | 4 | 1 | NULL | 3 | 1 |
+----------------+----------+--------+--------+--------+---------+
As per the table above, requirement 1 would only apply to any equipment having device type "1."
Requirement 2 would apply to all equipment having both manufacturer "2" AND building "2."
Requirement 3 would apply to all equipment having building "1" OR building "3"
Requirement 4 would apply to equipment having all of mfg_id "1" AND dev_id "3" AND building "1."
I am trying to write a query to give me a list of all equipment ids and all the associated frequency requirements based on the relationships defined in maint_equip. The problem I'm running into is handling the multiple joins. I have already tried:
SELECT equip.equip_id, maint.freq
FROM equip INNER JOIN
maint_equip ON equip.mfg_id = maint_equip.mfg_id
OR equip.mod_id = maint_equip.mod_id
OR equip.dev_id = maint_equip.dev_id
OR equip.bldg_id = maint_equip.bldg_id INNER JOIN
maint ON maint_equip.maint_id = maint.maint_id
but separating multiple joins using OR means that it is not accounting for the AND contingencies of each row. For example, maint_id 2 should only apply to equip_id 3 but ids 3 and 4 are both returned. If AND is used, then no rows are returned because none have a value for all columns.
Is it possible to join the tables in such a way to accomplish this or is there another way to structure the data?

Answer 1

If I get this right, when an equipment related ID in maint_equip is null, that should count as a match. Only if it isn't null, it must match the respective ID in equip. That is, you want to check if an ID in maint_equip is null or equal to its counterpart from equip.
SELECT e.equip_id,
m.freq
FROM equip e
INNER JOIN maint_equip me
ON (me.mfg_id IS NULL
OR me.mfg_id = e.mfg_id)
AND (me.mod_id IS NULL
OR me.mod_id = e.mod_id)
AND (me.dev_id IS NULL
OR me.dev_id = e.dev_id)
AND (me.bldg_id IS NULL
OR me.bldg_id = e.bldg_id)
INNER JOIN maint m
ON m.maint_id = me.main_id;

Answer 2

Try this:
( equip.mfg_id = maint_equip.mfg_id OR maint_equip.mfg_id is null )
AND( equip.mod_id = maint_equip.mod_id OR maint_equip.mod_id is null )
AND( equip.dev_id = maint_equip.dev_id OR maint_equip.dev_id is null )
AND( equip.bldg_id = maint_equip.bldg_id OR maint_equip.bldg_id is null )

Answer 3

Pay attention that your mod_id is always null. Otherwise query below goes through all your cases.
SELECT maint_equip.maint_id, equip.equip_id, maint.freq
FROM equip INNER JOIN
maint_equip ON (
(equip.mfg_id = maint_equip.mfg_id AND
equip.dev_id = maint_equip.dev_id AND
equip.bldg_id = maint_equip.bldg_id
) OR
(equip.mfg_id = maint_equip.mfg_id AND
maint_equip.dev_id is NULL AND
equip.bldg_id = maint_equip.bldg_id
) OR
(maint_equip.mfg_id is NULL AND
equip.dev_id = maint_equip.dev_id AND
maint_equip.bldg_id is NULL
) OR
(maint_equip.mfg_id is NULL AND
maint_equip.dev_id is NULL AND
equip.bldg_id = maint_equip.bldg_id
) )
INNER JOIN
maint ON maint_equip.maint_id = maint.maint_id
;

Answer 4

It seems to me that what you're actually looking for is the maintenance schedule that has the highest number of matches. You can get that by using a SUM with a series of CASE expressions to get the count of matching columns.
Then you have to account for ties where there are multiple maint_id values that match an equal number of times. For the example below, I opted to use maintenance frequency as the tie breaker, favoring more frequent maintenance over less frequent maintenance.
Rextester link with data set up: https://rextester.com/VISR88105
The ROW_NUMBER in the ORDER BY clause sorts the results by number of column matches (the nutty SUM/CASE combo) in descending order to get the most matches first, and then by maintenance frequency in ascending order to favor more frequent maintenance. Easy to reverse that with a DESC if you like. Then the TOP (1) WITH TIES limits the result set to all of the rows where ROW_NUMBER evaluates to 1.
The code:
SELECT TOP (1) WITH TIES
e.equip_id,
m.maint_id,
m.freq
FROM
#maint as m
JOIN
#maint_equip as me
ON
m.maint_id = me.maint_id
JOIN
#equip as e
ON
e.mfg_id = COALESCE(me.mfg_id, e.mfg_id)
AND
e.mod_id = COALESCE(me.mod_id, e.mod_id)
AND
e.dev_id = COALESCE(me.dev_id, e.dev_id)
AND
e.bldg_id = COALESCE(me.bldg_id, e.bldg_id)
GROUP BY
e.equip_id,
m.maint_id,
m.freq
ORDER BY
ROW_NUMBER() OVER (PARTITION BY e.equip_id ORDER BY (
SUM(
(CASE WHEN e.mfg_id = me.mfg_id THEN 1 ELSE 0 END) +
(CASE WHEN e.mod_id = me.mod_id THEN 1 ELSE 0 END) +
(CASE WHEN e.dev_id = me.dev_id THEN 1 ELSE 0 END) +
(CASE WHEN e.bldg_id = me.bldg_id THEN 1 ELSE 0 END)) ) DESC, m.freq )
Results:
+----------+----------+------+
| equip_id | maint_id | freq |
+----------+----------+------+
| 1 | 4 | 3 |
| 2 | 4 | 3 |
| 3 | 2 | 12 |
| 4 | 1 | 6 |
+----------+----------+------+