SQL query to reconstruct inherited EAV model - sql

Question

I have 5 tables in my database representing an inherited EAV model:
CREATE TABLE AttributeNames
("ID" int, "Name" varchar(8))
;
INSERT INTO AttributeNames
("ID", "Name")
VALUES
(1, 'Color'),
(2, 'FuelType'),
(3, 'Doors'),
(4, 'Price')
;
CREATE TABLE MasterCars
("ID" int, "Name" varchar(10))
;
INSERT INTO MasterCars
("ID", "Name")
VALUES
(5, 'BMW'),
(6, 'Audi'),
(7, 'Ford')
;
CREATE TABLE MasterCarAttributes
("ID" int, "AttributeNameId" int, "Value" varchar(10), "MasterCarId" int)
;
INSERT INTO MasterCarAttributes
("ID", "AttributeNameId", "Value", "MasterCarId")
VALUES
(100, 1, 'Red', 5),
(101, 2, 'Gas', 5),
(102, 3, '4', 5),
(102, 4, '$100K', 5),
(103, 1, 'Blue', 6),
(104, 2, 'Diesel', 6),
(105, 3, '3', 6),
(106, 4, '$80k', 6),
(107, 1, 'Green', 7),
(108, 2, 'Diesel', 7),
(109, 3, '5', 7),
(110, 4, '$60k', 7)
;
CREATE TABLE LocalCars
("ID" int, "MasterCarId" int)
;
INSERT INTO LocalCars
("ID", "MasterCarId")
VALUES
(8, '5'),
(9, '6'),
(10, NULL)
;
CREATE TABLE LocalCarAttributes
("ID" int, "AttributeNameId" int, "Value" varchar(6), "LocalCarId" int)
;
INSERT INTO LocalCarAttributes
("ID", "AttributeNameId", "Value", "LocalCarId")
VALUES
(43, 1, 'Yellow', 8),
(44, 3, '6', 9),
(45, 1, 'Red', 10),
(46, 2, 'Gas', 10),
(47, 3, '2', 10),
(48, 4, '$60k', 10)
;
I can retrieve all of master car attributes as follows:
SELECT MC.ID, MCA.AttributeNameId, MCA.Value
FROM MasterCars MC
left join MasterCarAttributes MCA on MC.ID = MCA.MasterCarId
order by MC.ID;
Likewise, I can retrieve all of the local car attributes as follows:
SELECT LC.ID, LCA.AttributeNameId, LCA.Value
FROM LocalCars LC
left join LocalCarAttributes LCA on LC.ID = LCA.LocalCarId
order by LC.ID;
If LocalCars.MasterCarId is not NULL, then that local car can inherit the attributes of that master car. A local car attribute with the same AttributeNameId overrides any master attribute with the same AttributeNameId.
So given the data above, I have 3 local cars each with 4 attributes (color, fuelType, doors, price). Inherited attribute values in bold:
Local Car Id = 1 (Yellow, Gas, 4, $100K)
Local Car Id = 2 (Blue, Diesel, 6, $80k)
Local Car Id = 3 (Red, Gas, 2, $60k)
I'm trying to find the necessary joins required to join the two queries above together to give a complete set of local cars attributes, some inherited:
LocalCarId AttributeNameId Value
------------------------------------------
1 1 Yellow
1 2 Gas
1 3 4
1 4 $100K
2 1 Blue
2 2 Diesel
2 3 6
2 4 $80K
3 1 Red
3 2 Gas
3 3 2
3 4 $60K
or possibly even:
LocalCarId AttributeNameId LocalValue MasterValue
-------------------------------------------------------------
1 1 Yellow Red
1 2 NULL Gas
1 3 NULL 4
1 4 NULL $100K
2 1 NULL Blue
2 2 NULL Diesel
2 3 6 3
2 4 NULL $80K
3 1 Red NULL
3 2 Gas NULL
3 3 2 NULL
3 4 $60K NULL

Answer 1

The problem can be solved by performing a union on all of your local car attributes and master car attributes. Each record is marked with an [IsMasterAttribute] flag. The next step is then use the ROW_NUMBER() window function to rank each of the duplicate attributes. The final step is to only select attributes which has a rank of 1.
;WITH CTE_CombinedAttributes
AS
(
SELECT 1 AS IsMasterAttribute
,LC.ID
,MC.ID AS MasterCarId
,MCA.AttributeNameId
,MCA.Value
FROM MasterCars MC
LEFT OUTER JOIN MasterCarAttributes MCA on MC.ID = MCA.MasterCarId
INNER JOIN LocalCars LC ON LC.MasterCarId = MC.ID
UNION ALL
SELECT 0 AS IsMasterAttribute
,LC.ID
,LC.MasterCarId
,LCA.AttributeNameId
,LCA.Value
FROM LocalCars LC
LEFT OUTER JOIN LocalCarAttributes LCA on LC.ID = LCA.LocalCarId
)
,
CTE_RankedAttributes
AS
(
SELECT [IsMasterAttribute]
,[ID]
,[AttributeNameId]
,[Value]
,ROW_NUMBER() OVER (PARTITION BY [ID], [AttributeNameId] ORDER BY [IsMasterAttribute]) AS [AttributeRank]
FROM CTE_CombinedAttributes
)
SELECT [IsMasterAttribute]
,[ID]
,[AttributeNameId]
,[Value]
FROM CTE_RankedAttributes
WHERE [AttributeRank] = 1
ORDER BY [ID]
The second output is also possible by performing a simple pivot on the final result:
;WITH CTE_CombinedAttributes
AS
(
SELECT 1 AS IsMasterAttribute
,LC.ID
,MC.ID AS MasterCarId
,MCA.AttributeNameId
,MCA.Value
FROM MasterCars MC
LEFT OUTER JOIN MasterCarAttributes MCA on MC.ID = MCA.MasterCarId
INNER JOIN LocalCars LC ON LC.MasterCarId = MC.ID
UNION ALL
SELECT 0 AS IsMasterAttribute
,LC.ID
,LC.MasterCarId
,LCA.AttributeNameId
,LCA.Value
FROM LocalCars LC
LEFT OUTER JOIN LocalCarAttributes LCA on LC.ID = LCA.LocalCarId
)
,
CTE_RankedAttributes
AS
(
SELECT [IsMasterAttribute]
,[ID]
,[AttributeNameId]
,[Value]
,ROW_NUMBER() OVER (PARTITION BY [ID], [AttributeNameId] ORDER BY [IsMasterAttribute]) AS [AttributeRank]
FROM CTE_CombinedAttributes
)
SELECT [ID]
,[AttributeNameId]
,MAX(
CASE [IsMasterAttribute]
WHEN 0 THEN [Value]
END
) AS LocalValue
,MAX(
CASE [IsMasterAttribute]
WHEN 1 THEN [Value]
END
) AS MasterValue
FROM CTE_RankedAttributes
GROUP BY [ID], [AttributeNameId]
ORDER BY [ID]

Answer 2

SQL Fiddle Demo
SELECT LC."ID" as LocalCarID,
COALESCE(LCA."AttributeNameId", MCA."AttributeNameId") as "AttributeNameId",
COALESCE(LCA."Value", MCA."Value") as "Value"
FROM LocalCars LC
LEFT JOIN MasterCars MC
ON LC."MasterCarId" = MC."ID"
LEFT JOIN MasterCarAttributes MCA
ON MC."ID" = MCA."MasterCarId"
LEFT JOIN LocalCarAttributes LCA
ON ( MCA."AttributeNameId" = LCA."AttributeNameId"
OR MCA."AttributeNameId" IS NULL)
-- This is the important part
-- Try to join with a MasterAtribute otherwise use the Car Atribute.
AND LC."ID" = LCA."ID"
OUTPUT
| LocalCarID | AttributeNameId | Value |
|------------|-----------------|--------|
| 1 | 1 | Blue |
| 1 | 2 | Gas |
| 2 | 1 | Green |
| 2 | 2 | Diesel |