PL/SQL looping 'back' through the records - oracle

Question

I have a problem i hope you can help to resolve.
I have a table named PRODUCT:
Product_ID NOT NULL NUMBER(10)
TARGET_PRODUCT VARCHAR2(10)
SOURCE_PRODUCT VARCHAR2(10)
So each target_product made out of source_product(except the first one - first one just has target_product and source_product is null)
I need to find the fist source_product for given target_product.
I need to go 'back' in a loop until source product is null.
Is there a solution for this scenario?
Thanks in advance

Answer 1

It is not a loop you need, but hierarchical query. Have a look at the following example based on Scott's EMP table.
This is its contents; employees are displayed hierarchically, showing who's whose boss:
SQL> select level,
2 lpad(' ', level * 2, ' ') || e.ename name
3 from emp e
4 start with e.mgr is null
5 connect by prior e.empno= e.mgr;
LEVEL NAME
---------- ---------------
1 KING
2 JONES
3 SCOTT
4 ADAMS
3 FORD
4 SMITH
2 BLAKE
3 ALLEN
3 WARD
3 MARTIN
3 TURNER
3 JAMES
2 CLARK
3 MILLER
14 rows selected.
SQL>
As you want to start from the middle of the table (for example, starting from SMITH), you'd "reverse" it:
SQL> select level lvl,
2 lpad(' ', level * 2, ' ') || e.ename name
3 from emp e
4 start with e.ename = 'SMITH'
5 connect by prior e.mgr= e.empno;
LVL NAME
---------- ---------------
1 SMITH
2 FORD
3 JONES
4 KING
SQL>
Finally, using that query as a CTE (or a subquery, if your Forms version doesn't support CTEs), fetch the one whose name has the highest level:
SQL> with temp as
2 (select level lvl,
3 lpad(' ', level * 2, ' ') || e.ename name
4 from emp e
5 start with e.ename = 'SMITH'
6 connect by prior e.mgr= e.empno
7 )
8 select trim(t.name) name
9 from temp t
10 where t.lvl = (select max(t1.lvl) from temp t1);
NAME
---------------
KING
SQL>
Or, even better, using connect_by_isleaf:
SQL> select e.ename name
2 from emp e
3 where connect_by_isleaf = 1
4 start with e.ename = 'SMITH'
5 connect by prior e.mgr= e.empno;
NAME
---------------
KING
SQL>

Answer 2

You may try this, it will return the base target_product for a given target_product:
select * from (
select PRODUCT_ID, TARGET_PRODUCT, SOURCE_PRODUCT
from PRODUCT
start with target_product = '<your target product>'
connect by prior SOURCE_PRODUCT = TARGET_PRODUCT
)
where SOURCE_PRODUCT is null;