SQL select merge two columns into one

Question

I have four tables:

Table A:

ID | B_ID
----------
 1 |  5
 2 |  6
 3 |  7
 4 |  8

Table B:

B_ID
-----
 5
 6
 7
 8

Table C:

C_ID | C_Name
--------------
 5   | Alpha
 6   | Beta

Table D:

D_ID | D_Name
--------------
 7   | Delta
 8   | Gamma

Note, that the values in Table B can come from either Table C or Table D.

I now need a query which shows ID from Table A and a second column called Name which consists of the corresponding names based on the B_ID column of Table B.

The expected result should look like:

ID | Name
----------
 1 |  Alpha
 2 |  Beta
 3 |  Delta
 4 |  Gamma

What I tried is this query:

SELECT *
FROM B
LEFT OUTER JOIN C
ON B_ID = C_ID
LEFT OUTER JOIN D
ON B_ID = D_ID

This yields:

B_ID | C_ID  | C_Name | D_ID | D_Name
-------------------------------------
 5   |  5    | Alpha  | Null | Null
 6   |  6    | Beta   | Null | Null
 7   |  Null | Null   | Null | Delta
 8   |  Null | Null   | Null | Gamma

However, I still have two issues:

1. I need to merge the names into a single column (see the expected result above)
2. It needs to be a subquery of a SELECT based on Table A in order to show the ID column of Table A.

Answer 1

Here's one option using a subquery with union all:

select a.id, b.name
from tablea a
   join (select id, name from tablec 
         union all select id, name from tabled) b on a.B_ID = b.id