Oracle SQL Conditional Outer Join using case statement

Question

Lets say that I have three tables:

1. TableA with columns TableAID (PK), Desc Nullable

2. TableB with columns TableBID (PK), TableAID(FK) Nullable

3. TableC with columns TableCID (PK), TableBID (FK) Nullable, 
   TableAID (FK) Nullable, Start_Date, End_Date

I need to return the Desc in Table A if TableC.TableBID is not null then use the TableAID(FK) in TableB to retrieve Desc else use TableAID (FK) in TableC to retrieve Desc

Note: It's possible both TableC.TableBID or TableC.TableAID can be null. In all cases I still must be able to return the other columns in TableC.

Here is my code:

Select ta.desc, tc.start_date, tc.end_date
from TableC tc
Left outer join TableB tb
on case
when tc.TableBID  is not null then (
           tc.TableBID = tb.TableBID
           Left outer join TableA ta
           on tb.TableAID = ta.TableAID 
           --my concern here is that tb.TableAID can be null. Will it still work?
)
else tc.TableAID = ta.TableAID --my concern here is that tc.TableAID can be null. 
--WIll it still work?

I'm also concern about syntax. If there is a better way to have a conditional join, please advise. I'm using oracle. This code will go into a view which will be used for a search procedure (that's why it has to return everything regardless of nulls). Thanks for your help.

Answer 1

You can put your CASE in the SELECT clause and join the tables accordingly, like this:

SELECT CASE
           WHEN tb.tableAID IS NOTNULL THEN tab.desc
           ELSE tac.desc
       END AS desc
       -- or better: NVL(tab.desc, tac.desc) AS desc
       , tc.start_date
       , tc.end_date
  FROM tableC tc
  JOIN tableB tb ON tc.tableBID = tb.tableBID
  LEFT JOIN tableA tab ON tab.tableAID = tb.tableAID
  LEFT JOIN tableA tac ON tac.tableAID = tc.tableAID