How to convert code from Postgres to Oracle - oracle

Question

I have a source table (T1):
ID1 | ID2
----------
1 | 2
1 | 5
4 | 7
7 | 8
9 | 1
I want to convert the data to this (T2):
ID1 | ID2 | LABEL
------------------
1 | 2 | 1
1 | 5 | 1
4 | 7 | 2
7 | 8 | 2
9 | 1 | 1
I found a solution for this in PostgreSQL:
with
recursive cte(id1, id2) as (
select id1, id2, 1 as level
from t
union all
select t.id1, cte.id2, cte.level + 1
from t join
cte
on t.id2 = cte.id1
)
select id1, id2,
dense_rank() over (order by grp) as label
from (select id1, id2,
least(min(id2) over (partition by id1), min(id1) over (partition by id2)) as grp,
level
from cte
) t
where level = 1;
I want to convert this code to Oracle. How I can convert this code from Postgres to Oracle?

Answer 1

Oracle 11.2 supports recursive CTEs. But it deviates from the standard in that the recursive keyword is not required (actually: must not be used). So if you remove the recursive keyword and get the definition of the CTE columns right the following should work. You also need to use something different than LEVEL as that is a reserved word either.
with cte (id1, id2, lvl) as (
select id1, id2, 1 as lvl
from t
union all
select t.id1, cte.id2, cte.lvl + 1
from t
join cte on t.id2 = cte.id1
)
select id1,
id2,
dense_rank() over (order by grp) as label
from (
select id1,
id2,
least(min(id2) over (partition by id1), min(id1) over (partition by id2)) as grp,
lvl
from cte
) t
where lvl = 1;
Here is an SQLFiddle example: http://sqlfiddle.com/#!4/deeb2/3
However I doubt that the original query was correct as you do not have a "starting condition" for the recursive CTE. The first part of the union retrieves all rows of the table. There should be a condition to restrict that to the "roots" of the hierarchy unless I'm mis-understanding the purpose of the query.
A recursive CTE can also be replaced with a CONNECT BY query, in your case this would be:
select id1, id2, level as lvl
from t
connect by prior id1 = id2;
You can combine that with the orginal query:
with cte (id1, id2, lvl) as (
select id1, id2, level as lvl
from t
connect by prior id1 = id2
)
select id1,
id2,
dense_rank() over (order by grp) as label
from (
select id1,
id2,
least(min(id2) over (partition by id1), min(id1) over (partition by id2)) as grp,
lvl
from cte
) t
where lvl = 1;
Although I think it should be the same, it seems that the hierarchy is traversed in a different order. Could be because the recursive CTE does a breadth first and the connect by a depth first recursion (or the other way round).
SQLFiddle example for the second version: http://sqlfiddle.com/#!4/deeb2/4