JOIN 2 Tables get Unique values from 2 columns - tsql

Question

I have a (i think) complicated problem, and have no idea how to do that in SQL (the whole day). I have turned the logic around a couple of times, and always something is missing.
There is a join between 2 tables that hold different FK references to a 3rd table.
How to join those 2 tables, so i am sure that all FK combinations are presented, and all are unique?
I need to have the 2 FK columns in one, so i can later join to 3rd. nulls are possible. Group by not possible, since i need to know where the record is from (need Id_1 and Id_2 in the result)
here the sample code:
DECLARE #T1 TABLE (Id int, CommonId int, FK_Id_1 int)
DECLARE #T2 TABLE (Id int,CommonId int, FK_Id_2 int)
INSERT INTO #T1 VALUES (1,1,1)
INSERT INTO #T1 VALUES (2,1,2)
INSERT INTO #T1 VALUES (3,2,3)
INSERT INTO #T1 VALUES (4,3,NULL)
INSERT INTO #T1 VALUES (5,4,NULL)
INSERT INTO #T2 VALUES (11,1,1)
INSERT INTO #T2 VALUES (12,2,2)
INSERT INTO #T2 VALUES (13,2,3)
INSERT INTO #T2 VALUES (14,4,5)
SELECT t1.Id as Id_1,t2.Id as Id_2, t1.CommonId, t1.FK_Id_1, t2.FK_Id_2,
COUNT(t1.FK_Id_1) OVER (PARTITION BY t1.FK_Id_1) AS T1_RANK,
COUNT(t2.FK_Id_2) OVER (PARTITION BY t2.FK_Id_2)AS T2_RANK
FROM #T1 t1
FULL JOIN #T2 t2 on t1.CommonId = t2.CommonId
ORDER BY CommonId
This query is returning this:
Id_1 Id_2 CommonId FK_Id_1 FK_Id_2 T1_RANK T2_RANK
----------- ----------- ----------- ----------- ----------- ----------- -----------
1 11 1 1 1 1 2
2 11 1 2 1 1 2
3 12 2 3 2 2 1
3 13 2 3 3 2 1
4 NULL 3 NULL NULL 0 0
5 14 4 NULL 5 0 1
and i need somehow to make it look like this:
Id_1 Id_2 CommonId FK_Id
----------- ----------- ----------- -----------
1 11 1 1
2 11 1 2
3 12 2 2
3 13 2 3
4 NULL 3 NULL
5 14 4 5
I did something like SELECT COALESCE(FK_Id_1,FK_Id_2) AS FK_Id but this is always selecting T1 with priority. I am thinking of some way to switch the priority depending of the duplicated values.
i have a ugly solution that looks like this, but am looking for a better ideas.
;WITH tmp as (
SELECT t1.Id as Id_1,t2.Id as Id_2, t1.CommonId, t1.FK_Id_1, t2.FK_Id_2,
COUNT(t1.FK_Id_1) OVER (PARTITION BY t1.FK_Id_1) AS T1_RANK,
COUNT(t2.FK_Id_2) OVER (PARTITION BY t2.FK_Id_2)AS T2_RANK
FROM #T1 t1
FULL JOIN #T2 t2 on t1.CommonId = t2.CommonId)
SELECT Id_1, Id_2, CommonId,
CASE
WHEN T1_RANK > T2_RANK THEN COALESCE(FK_Id_2,FK_Id_1)
WHEN T2_RANK > T1_RANK THEN COALESCE(FK_Id_1,FK_Id_2)
END AS FK_Id
FROM tmp
ORDER BY CommonId
I don't know if i explained the whole situation correctly, i must join the tables, because i have other columns coming only from T1 and T2 ( can not UNION->DISTINCT - this will select also the NULLs)

Answer 1

just pick the CommonId, and then full join to both tables.
the query below matches 100% to your desired result.
;WITH cte AS (
SELECT CommonId FROM #T1
UNION SELECT CommonId FROM #T2
)
SELECT t1.Id AS Id_1, t2.Id AS Id_2, cte.CommonId, ISNULL(t2.FK_Id_2, t1.FK_Id_1) AS FK_Id
FROM cte
FULL OUTER JOIN #T1 t1 ON cte.CommonId = t1.CommonId
FULL OUTER JOIN #T2 t2 ON cte.CommonId = t2.CommonId
Beware that that the results of the FK_Id changes with the column order in the ISNULL in the predicate
ISNULL(t2.FK_Id_2, t1.FK_Id_1) is not the same as ISNULL(t1.FK_Id_1, t2.FK_Id_2)
In my opinion this alternative version fits better your request, since it's taking both options of FK.
;WITH cte AS (
SELECT CommonId FROM #T1
UNION SELECT CommonId FROM #T2
)
SELECT t1.Id AS Id_1, t2.Id AS Id_2, cte.CommonId, ISNULL(t2.FK_Id_2, t1.FK_Id_1) AS FK_Id--, cte.CommonId, *
FROM cte
FULL OUTER JOIN #T1 t1 ON cte.CommonId = t1.CommonId
FULL OUTER JOIN #T2 t2 ON cte.CommonId = t2.CommonId
UNION
SELECT t1.Id AS Id_1, t2.Id AS Id_2, cte.CommonId, ISNULL(t1.FK_Id_1, t2.FK_Id_2) AS FK_Id--, cte.CommonId, *
FROM cte
FULL OUTER JOIN #T1 t1 ON cte.CommonId = t1.CommonId
FULL OUTER JOIN #T2 t2 ON cte.CommonId = t2.CommonId