The typical hierarchical query is you want to select your boss, and the boss of your boss, etc.
It could look like
select prior ename ename, ename mgr
connect by prior mgr=empno
start with ename=’SCOTT’;
I start with Scott and the hierarchy is built. I can use the pseudo column LEVEL in hierarchical queries.
One of the biggest problem in hierarchical queries is ORA-01436: CONNECT BY loop in user data.
If you are your own boss, or if you are the boss of your boss, then you have built a cycle. It is probably not wished to have this relation, but it cannot be enforced by a constraint and before 10g, it was difficult to detect. In 10g, you have a new clause, CONNECT BY NOCYCLE, which detect cycles and give flag.
Here I want to present an alternative way of using hierarchies.
I have five boxes of five different sizes. Tiny up to 5 liters. Small up to 10 liters. Medium up to 15 liters. Big up to 20 liters. Hudge up to 25 liters.
I have 3 fluids, and I want to find the smallest boxes.
Yeah! very easy, you do select fluids.volume, min(box.capacity) from fluids, box where capacity>=volume group by volume, do not you?
Ok, but I do not want to mix the fluids! So I need 3 different containers.
This means, I will start with the first product, find the smallest box, go to the second, find the smallest free box, and so on.
Hmm… It seems an impossible task with analytics, least, min, keep, lag, lead, ???
Ok, I am going to build a hierarchy, based on volume > prior volume and capacity > prior capacity.
Then I do a min with the sys_connect_by_path function.
select max(sys_connect_by_path(volume,’/')) volume_path,
(select row_number() over (order by volume) r, count(*) over () c, volume from fluids)
connect by capacity > prior capacity and r > prior r;
There is also one more CONNECT I would like to briefly mention, is the connect without prior.
select level from dual connect by level<11;
It is a special way of creating pivot tables, but it is fairly dangerous, and could make your session / database hang, depending on your oracle version.