Join based on the value to different table - sql-server

Question

Hi everyone I have a table TableC which saves primary key values from two different tables TableA and TableB. because they are primary keys from two tables I could have end up with duplicates in that tableC so when Storing values I prefixed the Primary keys with a short text to differentiate that which value is coming from which table.
Now I would like to Join this tableC with TableA and TableB to get the data from tableA and TableB
TableC :
ID_Column
1A
1B
2A
TableA:
ID_Column | Data
1 | data A 1
2 | data A 2
3 | data A 3
TableB:
ID_Column | Data
1 | data B 1
2 | data B 2
3 | data B 3
This is what I have been trying to do
select C.ID_Column, data
from tableC C
inner join tableA A
on A.ID_Column = left(C.ID_Column, 1)
inner join tableB B
on B.ID_Column = left(C.ID_Column, 1)
this will return data from both tables I want to return data from table b when ID_Column has B in the end and want to return data from tableA when ID_Column has A in the end
Thank you in advance.

Answer 1

Well, TableC should really have at least 2 columns, one for the id and one to identify to which table it belongs. Anyway, this could be done this way:
SELECT C.ID_Column,
ISNULL(A.Data,B.Data) Data
FROM TableC C
LEFT JOIN TableA A
ON LEFT(C.ID_Column,LEN(C.ID_Column)-1) = A.ID_Column
AND RIGHT(C.ID_Column,1) = 'A'
LEFT JOIN TableB B
ON LEFT(C.ID_Column,LEN(C.ID_Column)-1) = B.ID_Column
AND RIGHT(C.ID_Column,1) = 'B'
The results are:
╔══════════╦══════════╗
║ D_COLUMN ║ DATA ║
╠══════════╬══════════╣
║ 1A ║ data A 1 ║
║ 1B ║ data B 1 ║
║ 2A ║ data A 2 ║
╚══════════╩══════════╝
And here is a demo for you to try.

Answer 2

You'll obviously need to tweak this a bit, but could you consider refactoring table C just a bit. Seperate the two out and this will eliminate the calculation in the join (not a good performance tactic).
--Create Table A
IF OBJECT_ID('tempdb..#TableA') IS NOT NULL DROP TABLE #TableA
CREATE TABLE #TableA (ID_COLUMN INT, DATA VARCHAR(max))
INSERT INTO #TableA(ID_COLUMN,DATA) VALUES (1,'data A 1'),(2,'data A 2'),(3,'data A 3')
--Create Table B
IF OBJECT_ID('tempdb..#TableB') IS NOT NULL DROP TABLE #TableB
CREATE TABLE #TableB (ID_COLUMN INT, DATA VARCHAR(max))
INSERT INTO #TableB (ID_COLUMN,DATA) VALUES (1,'data B 1'),(2,'data B 2'),(3,'data B 3')
--Create Table C
IF OBJECT_ID('tempdb..#TableC') IS NOT NULL DROP TABLE #TableC
CREATE TABLE #TableC (ID_COLUMN INT, ORIGIN_TABLE CHAR(1))
INSERT INTO #TableC (ID_COLUMN, ORIGIN_TABLE) VALUES (1,'A'), (1,'B'),(2,'A')
SELECT A.ID_COLUMN, A.DATA
FROM #TableC AS C
INNER JOIN #TableA AS A ON (C.ID_COLUMN = A.ID_COLUMN)
WHERE C.ORIGIN_TABLE = 'A'
UNION ALL
SELECT B.ID_COLUMN, B.DATA
FROM #TableC AS C
INNER JOIN #TableB AS B ON (C.ID_COLUMN = B.ID_COLUMN)
WHERE C.ORIGIN_TABLE = 'B'