Avoid UNION ALL in SQL Server

Question

I have the following (simplified) problem. I have 3 Tables: MainTable, TableId1 and TableId2. They look like this

MainTable

+-------+------+-----+
| Type  | Id1  | Id2 |
+-------+------+-----+
| a     | 0    |   3 |
| b     | 1    |   0 |
+-------+------+-----+

TableId1

+-----+------+
| Id1 | code |
+-----+------+
|   1 | abc  |
+-----+------+

TableId2

+-----+------+
| Id2 | code |
+-----+------+
|   3 | xyz  |
+-----+------+

I want to obtain the following result Set:

+-------+------+
| Type  | code |
+-------+------+
| a     | xyz  |
| b     | abc  |
+-------+------+

I am right now using a UNION ALL, but it is not elegant and becomes a pain when my query becomes more complex and I have to edit it etc.

There has to me a way to avoid UNION ALL here I hope, can anyone show me?

Thanks!

Answer 1

select type,isnull(tid1.code,tid2.code) as Code
from MainTable mt
left join TableId1 tid1 on mt.Id1=tid1.Id1
left join TableId2 tid2 on mt.Id2=tid2.Id2

SQL Fiddle to test Query

Answer 2

Use LEFT OUTER JOIN and use where clause for Id1 and Id2 != 0. Will post query in a minute.

Answer 3

You could try:

SELECT 
    mt.Type
    ,t1.Code
    ,t2.Code
FROM 
    MainTable  mt
        LEFT OUTER JOIN TableId1 t1
        ON mt.Id1 = t1.Id1
            LEFT OUTER JOIN TableId2 t2
            ON mt.Id2 = t1.Id2

However this would give you:

+-------+------+------+
| Type  | code | code |
+-------+------+------+
| a     | null | xyz  |
| b     | abc  | null |
+-------+------+------+

which may or may not be ideal for your scenario.

Answer 4

Define a view that gets data from all those 3 tables (using UNION ALL or not). This way you'll have to modify a single piece of code while your query "becomes complex".