MySQL多级嵌套案例什么时候优先？

Question

I am trying to wrap my mind around this little problem I've run into.

I am joining multiple tables containing vehicle information (so make, model, type, subtype, part numbers and part titles) in order to go through my "title override" table that might (or might not) contain entries where certain vehicle parts would have their title overwritten in certain cases, for certain vehicle makes, models, types or subtypes.

As in, think of using standard oil for all BMW cars, and then for BMW M5 there is an exception that overrides that standard oil with another premium oil. Or say standard oil is being used for all cars UNLESS otherwise specified in this table. And the priority goes down the list:

• specific model (highest priority)
• specific make (2nd highest priority)
• subtype (3rd highest priority)

So the override table ("ov") looks something like this:

id | type_id | make_id | subtype_id | model_id | part_id | part_override
1  |   2     |    0    |    0       |    0     |   33    |  Global-Replacement-Oil
2  |   2     |    1    |    0       |    0     |   33    |  Yamaha-Premium-Oil
3  |   2     |    0    |    2       |    0     |   33    |  Global-Dirtbike-Oil
4  |   2     |    1    |    4       |    0     |   33    |  Yamaha-Streetbike-Oil
5  |   2     |    0    |    0       |   199    |   33    |  Yamaha-R6-2015-Oil

** type_id = 2 is for motorcycles, type_id = 3 is for cars, for example

** if make_id, subtype_id or model_id are = 0 that means that override should apply to all makes, subtypes and models, unless value <> 0 is specified.

And for that part_id = 33, I only need to display it once in my results, not multiple times, depending on which model is queried in the application and where it fits in that "matrix".

So if I am running this query on Yamaha R6 2015 edition and need to pull the code/title for that part_id=33 for that model, "model_id" column has the highest priority over all the other ones, so I'll ignore all the other ones and show title for record #5.

If I however pick another Yamaha Street bike, the highest priority match I'd get would be record #4 (as it's got both make_id and subtype_id defined).

Or if I pick a random Dirtbike, I'll get a match on record #3 (on subtype_id), so that code/title will display instead.

And then if I pick any Yamaha bike, I'll get a match on record #2 (on make_id).

And at last, if none of those are specified and I just pick a random bike, it will show record #1 instead, as that will make a match on type_id alone (bikes = 2).

This is the query that I have so far (or part of it):

SELECT DISTINCT
CASE 
  WHEN p.part_id = '72' AND ov.part_override != "" AND ov.type_id = p.type_id AND 
    (CASE 
      WHEN ov.model_id IS NOT NULL 
      THEN ov.model_id = "$MODEL_ID"
      ELSE 
        CASE 
          WHEN ov.make_id != 0 
          THEN ov.make_id = m.make_id
          ELSE
            CASE 
              WHEN ov.subtype_id != 0 
              THEN ov.subtype_id = st.subtype_id 
              ELSE 1 
            END
         END
      END)
  THEN ov.part_override
  ELSE p.part_number 
END part_number,
ov.make_id,
....
FROM parts p ON ...
INNER JOIN makes m ON p.make_id = m.make_id
INNER JOIN ... 
(just joining a bunch of other separate tables that contain all types,
subtypes, models and so on, respectively - irrelevant for the logic above)

What I am getting with this query is all the parts that apply to a specific model listed, so if I run it for a specific model like Yamaha R6 2015 I'll get the records #1,2 and 5 come up. But I just need record #5 to show up, as explained above, in the order of importance and priority.

If I do a GROUP BY part_id or something like that then I only get one record showing, but it's not necessarily the right one, in terms of highest priority.

What am I missing?

Or how can I cascade this whole importance/priority check down that query, so that while going through all the parts records in other joined tables and filtering it based on this override ("ov") table it only spits out the highest priority record, based on some sort of a waterfall or cascade rule?

Not sure how to write that, or if it's even possible.

Or if I have to run it as a recurring stored procedure of sorts, that's fine too.

Thanks!

Answer 1

Without seeing more accurately your other table joins, nor the actual table name you have here, you might want something with multiple left-joins to the override table, then pull COALESCE based on the first FOUND value. Something like..

select
      ...
      coalesce( P_OV.part_override, M_OV.part_override,
             MOD_OV.part_override, P.PartName ) as FinalPartName
   from
      Parts P
         left join PartsOverride P_OV
            on p.id = P_OV.part_id
         join make m
            on p.make_id = m.make_id
            left join PartsOverride M_OV
               on m.make_id = M_OV.make_id
         join model mo
            on p.model_id = mo.model_id
            left join PartsOverride MOD_OV
               on m.make_id = MOD_OV.make_id

Then, you can change the order of priority of which "OV" version you want within the coalesce(). So if Model is higher priority than make:

  coalesce( MOD_OV.part_override, M_OV.part_override,
         P_OV.part_override, P.PartName ) as FinalPartName

Whichever field is NOT NULL FIRST WINS.