JOIN / MERGE several tables with same key - sql-server

Question

I have several tables that I have joined like this:
SELECT *
FROM tableA AS A
FULL OUTER JOIN tableB AS B
ON B.KEY = A.KEY
FULL OUTER JOIN tableC AS C
ON C.KEY = A.KEY
FULL OUTER JOIN tableD AS D
ON D.KEY = A.KEY
This works fine, but i sometime get separate rows where the key does not exist in all tables.
Example
WITH tableA([KEY], payloadColumn) AS
(
SELECT 1, 'var_A1'
), tableB([KEY], payloadColumn) AS
(
SELECT 2, 'var_B2'
), tableC([KEY], payloadColumn) AS
(
SELECT 1, 'var_C1'
), tableD([KEY], payloadColumn) AS
(
SELECT 2, 'var_D2'
)
SELECT * FROM tableA as A
FULL OUTER JOIN tableB as B ON B.[KEY] = A.[KEY]
FULL OUTER JOIN tableC as C ON C.[KEY] = A.[KEY]
FULL OUTER JOIN tableD as D ON D.[KEY] = A.[KEY]
Returns
+------+---------------+------+---------------+------+---------------+------+---------------+
| KEY | payloadColumn | KEY | payloadColumn | KEY | payloadColumn | KEY | payloadColumn |
+------+---------------+------+---------------+------+---------------+------+---------------+
| 1 | var_A1 | NULL | NULL | 1 | var_C1 | NULL | NULL |
| NULL | NULL | 2 | var_B2 | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | NULL | 2 | var_D2 |
+------+---------------+------+---------------+------+---------------+------+---------------+
But i need them all to be in the same row in the result (not two rows for 2). How can I do this?

Answer 1

That's my proposed solution based on the draft in a previous version of OP, and their last edit:
CREATE VIEW ABCD
AS
SELECT [KEY] FROM tableA
UNION
SELECT [KEY] FROM tableB
UNION
SELECT [KEY] FROM tableC
UNION
SELECT [KEY] FROM tableD
Creating a view may not be an option; in this case rewrite as a CTE.
At least in this way, the execution plan will continue to use indexes (assuming [KEY] is a PK or indexed).
SELECT
K.[KEY] as sameKey,
A.payloadColumn,
B.payloadColumn,
C.payloadColumn,
D.payloadColumn
FROM ABCD K
LEFT JOIN tableA A ON A.[KEY]=K.[KEY]
LEFT JOIN tableB B ON B.[KEY]=K.[KEY]
LEFT JOIN tableC C ON C.[KEY]=K.[KEY]
LEFT JOIN tableD D ON D.[KEY]=K.[KEY]
If this got your idea wrong, please elaborate.

Answer 2

You can use COALESCE
WITH tableA([KEY], payloadColumn)
AS (SELECT 1,
'var_A1'),
tableB([KEY], payloadColumn)
AS (SELECT 2,
'var_B2'),
tableC([KEY], payloadColumn)
AS (SELECT 1,
'var_C1'),
tableD([KEY], payloadColumn)
AS (SELECT 2,
'var_D2')
SELECT COALESCE(A.[KEY], B.[KEY], C.[KEY], D.[KEY]) AS [KEY],
A.payloadColumn,
B.payloadColumn,
C.payloadColumn,
D.payloadColumn
FROM tableA AS A
FULL OUTER JOIN tableB AS B
ON B.[KEY] = A.[KEY]
FULL OUTER JOIN tableC AS C
ON C.[KEY] = COALESCE(A.[KEY], B.[KEY])
FULL OUTER JOIN tableD AS D
ON D.[KEY] = COALESCE(A.[KEY], B.[KEY], C.[KEY]);
Returns
+-----+---------------+---------------+---------------+---------------+
| KEY | payloadColumn | payloadColumn | payloadColumn | payloadColumn |
+-----+---------------+---------------+---------------+---------------+
| 1 | var_A1 | NULL | var_C1 | NULL |
| 2 | NULL | var_B2 | NULL | var_D2 |
+-----+---------------+---------------+---------------+---------------+
If all the tables have a covering index on [KEY] INCLUDE (payloadColumn) the execution plan for this can just chain together merge joins.