Group select statements together by common column in sql-server - sql

Question

I need to combine two tables together based on a common column. Normally I would just use an inner join on the specific column (lets call it parentID), but I need the results to be in seperate rows.
Table A:
ID, ...
Table B:
ID, ParentID, SomeColB, ...
Table C:
ID, ParentID, SomeColC, ...
ParentID points to the ID of table A. The result should look as follows:
ParentID ID_A ID_B SomeColB SomeColC
1 10 20 'VAL_B1' NULL
1 10 20 NULL 'VAL_C1'
2 11 21 'VAL_B2' NULL
2 11 21 NULL 'VAL_C2'
...
So I want to alternate between selecting values from Table B and C and leave the remaining columns on null. How would I do that?
I tried joining them together but this results in results being put into a single row.
EDIT: Both Table B and C have a 1-n relationship to table A (one entry in table a can be referenced from multiple entries in table B and C). Table B and C don't reference each other and are completely independent of eachother.

Answer 1

Would something like this work for you? I've used a UNION to get both sets of data per ParentID:
SELECT
*
FROM (
SELECT
ParentID,
ID_A,
ID_B,
SomeCol B,
NULL AS SomeColC
FROM
TableA
UNION
SELECT
ParentID,
ID_A,
ID_B,
NULL AS SomeColB,
SomeColC
FROM
TableB
)
ORDER BY
ParentID,
SomeColB,
SomeColC

Answer 2

You should use union operator
SELECT IDA, ParentIDA, SomeColA FROM first_table
UNION
SELECT IDB, ParentIDB, SomeColB FROM second_table
UNION will skip the duplicate recored
if you want to show the duplicate records you should use UNION ALL operator

Answer 3

Looks like what you really want is a LEFT OUTER JOIN.
A slimmed down version of your select with the pertinent fields would look like this...
select a.ID as ParentID, b.SomeCol as SomeColB, c.SomeCol as SomeColC
from tableA a
left outer join tableB b
on b.ID = a.ID
left outer join tableC c
on c.ID = a.ID
;
Left outer joins include non-matching rows from the left table in the join, providing NULL values for the fields coming from the unmatched records in the table to the right in the join.

Answer 4

Bit of a stab in the dark, but it gets you the result set you want based on the sample data:
WITH A AS(
SELECT ID
FROM (VALUES(1),(2)) V(ID)),
B AS(
SELECT V.ID,
V.ParentID,
V.ColB
FROM (VALUES(1, 10,'Val_B1'),
(2,11,'Val_B2'))V(ParentID,ID, ColB)),
C AS(
SELECT V.ID,
V.ParentID,
V.ColC
FROM (VALUES(1,20,'Val_C1'),
(2,21,'Val_C2'))V(ParentID,ID, ColC))
SELECT A.ID AS ParentID,
B.ID AS ID_A,
C.ID AS ID_B,
B.ColB,
C.ColC
FROM A
CROSS APPLY (VALUES('B'),('C'))V(T)
LEFT JOIN B ON A.ID = B.ParentID
AND V.T = 'B'
LEFT JOIN C ON A.ID = C.ParentID
AND V.T = 'C'
ORDER BY A.ID,
V.T;
DB<>fiddle

Answer 5

My guess it's a UNION of JOINs
SELECT A.ID AS ParentID, B.ID AS ID_B, null as ID_C, B.SomeColB, null as SomeColC --, ..
FROM A
JOIN B ON A.ID = B.ParentID
UNION
SELECT A.ID AS ParentID, null, c.ID as ID_C, null, C.SomeColC --, ..
FROM A
JOIN C ON A.ID = C.ParentID
ORDER BY ParentID, ID_B, ID_C;
To repeat ids wrap it with one more SELECT:
SELECT ParentID
, max(ID_B) OVER(PARTITION BY ParentID) AS ID_B
, max(ID_C) OVER(PARTITION BY ParentID) AS ID_C
, SomeColB, SomeColC --, --
FROM (
SELECT A.ID AS ParentID, B.ID AS ID_B, null as ID_C, B.SomeColB, null as SomeColC --, ..
FROM A
JOIN B ON A.ID = B.ParentID
UNION
SELECT A.ID AS ParentID, null, c.ID as ID_C, null, C.SomeColC --, ..
FROM A
JOIN C ON A.ID = C.ParentID) t
ORDER BY ParentID, ID_B, ID_C;