PL_SQL hard for me join - dynamic ID-s in second table

I tried to resolve this but I have no idea how to do it.

I have two tables, TABLE_S and TABLE_G. In TABLE_S I have an ID and in TABLE_G, I have two: ID1 and ID2.

TABLE_S     ID
TABLE_G     ID1    ID2

I want a left join, if there's no result for TABLE_G_ID1, then left join TABLE_G_ID2

Something like this:

SELECT * 
FROM TABLE_S
LEFT JOIN TABLE_G ON TABLE_S.ID = TABLE_G.ID1 
                  -- if no result then: ON TABLE_S.ID = TABLE_G.ID2

I tried this:

Oracle try second condition only if first one gave nothing in join

But it doesn't work for me, and there is full join, not left join.

Appreciate your suggestion!

Use AND and OR:

SELECT *
FROM   TABLE_S s
       LEFT OUTER JOIN TABLE_G g
       ON (  s.ID = g.ID1
          OR (
                 (s.ID <> g.ID1 OR s.ID IS NULL OR g.ID1 IS NULL)
             AND s.ID = g.ID2
             )
          );

(Which is exactly the same logic as the question you link.)

or use a CASE expression:

SELECT *
FROM   TABLE_S s
       LEFT OUTER JOIN TABLE_G g
       ON s.ID = CASE s.ID WHEN g.ID1 THEN g.ID1 ELSE g.ID2 END;

(Which is shorter to type but you may find that the SQL engine has difficulty using indexes on the columns due to them being inside the CASE expression.)