replace a union query involving a left join - oracle

Question

I have a left join and a Union .
I was thinking if it is possible to replace the union with a join
select t1.col1,t1.col2,t2.col1,
from t1
left join t2 on (t1.col1 = t2.col1)
union
select t3.col1,t3.col2,t2.col1
t2.col1,
from t3
left join t2 on (t3.col1 = t2.col1)
I am on Oracle 12c can this query be re-written without the union ?
I tried this but the results don't match the union query's results
select t1.col1,t3.col2,t2.col1,
from t1
left join t2 on (t1.col1 = t2.col1)
left join t3 on (t3.col1 = t2.col1 and t1.col1 = t2.col1)
Any ideas is this even possible ?

Answer 1

Here are a couple of alternatives for you:
Do the union of t1 and t3 first, before joining to t2:
select t.col1, t.col2, t2.col1
from (select col1, col2
from t1
union
select col1, col2
from t3) t
left outer join t2 on (t.col1 = t2.col1);
Use a full outer join of t1 and t3 first, before joining to t2:
select t.col1, t.col2, t2.col1
from (select coalesce(t1.col1, t3.col1) col1,
coalesce(t1.col2, t3.col2) col2
from t1
full outer join t3 on t1.col1 = t3.col1
and t1.col2 = t3.col2) t
left outer join t2 on (t.col1 = t2.col1);
N.B. Both are untested, since you neglected to provide any sample data in any of the three tables. You'll have to test these approaches yourself.
ETA: Also, I believe both these approaches should work for the example as given. If your real world scenario differs from that in your question, it's possible neither approach will be of use.
ETA2: Given you're wishing to tweak your query for readability, I'd go with my first proposed solution (assuming it works for you, of course!), since the union deduplicates the rows from tables t1 and t3 before joining to t2 - that way, you're reducing the work needed to do the join. Also, the union has less to check too (2 cols instead of 3).
It may have a union in it, but it's sure as heck nicer to read than my second suggestion! At least, in my opinion *{;-)