connect by prior giving incorrect results - oracle

Question

Assume I have 2 tables as shown below – PARENT and CHILD
create table parent
(
parent_id number,
parent_name varchar2(10),
cnt number
)
create table children
(
child_id number,
parent_id number,
name varchar2(10)
)
insert into parent values (1, 'A', 3);
insert into parent values (2, 'B', 2);
insert into children values (1, 1, 'AB');
insert into children values (1, 2, 'AC');
insert into children values (1, 3, 'AD');
insert into children values (2, 1, 'BA');
insert into children values (2, 2, 'BB');
The output has to be something like below:
Parent_ID Parent_Name Cnt Child_Names
1 A 3 AB, AC, AD
2 B 2 BA, BB
I have written the below query to achieve this. I don't know where it is going wrong, query seems to be fine but the output is not what is desired.
Please help me out as I am almost saturated debugging this.
select parent_id, parent_name, substr(max(sys_connect_by_path(child_name, ',')),2)
from
( select p.parent_id, p.parent_name, ch.child_name, row_number() over (partition by p.parent_id, p.parent_name order by ch.child_name) rn
from parent p, children ch
where p.parent_id = ch.parent_id
)
start with rn =1
connect by prior rn+1 = rn
group by parent_id, parent_name

Answer 1

You don't see hierartical data. It is simple master detail.
select p.parent_id, p.parent_name, p.cnt, c.children_names, c.real_count
from parent p left outer join
(select parent_id, listagg(name, ', ') within group (order by name) children_names,
count(*) real_count from children group by parent_id) c
on p.parent_id = c.parent_id
'AD' is not connected because it parent_id is 3.
Hm... listagg is a 11g feature.
The general solution before 11g was to use Tom Kyte's aproach:
http://www.sqlsnippets.com/en/topic-11591.html
And have a look at http://www.oracle-developer.net/display.php?id=515 for performance expectations
Ok, let's continue with "connect by" aproach.
First of all, lets reduce complexity by forgetting about parent table. We will be building right side of left outer join query above, but using connect by construction.
Second, I think you missed how parent_id and child_id are named. My guess is that your child_id is actually your parent_id in parent table. So name is wrong.
3 conditions to put all children in a row:
All children have the same child_id (parent.parent_id)
first one has parent_id = 1 (more like order_id)
every next one has parent_id +1 comparing to previous one
Let's put it all into query:
select c.*, level from children c
start with parent_id = 1 --2nd condition
connect by prior
c.parent_id = c.parent_id -1 --3rd condition
and prior child_id = child_id --1st condition
order by child_id, parent_id
Then you add your SYS_CONNECT_BY_PATH(name, ', '), get the last element and only then join it with parent table, but based on parent.parent_id = children.child_id

Answer 2

here is your query after correction, actually you did some mistake in column name of children table(wrote ch.child_name while it is ch.name or name)
select parent_id, parent_name,cnt,substr(max(sys_connect_by_path(child_name, ',')),2) child_name
from
( select p.parent_id, p.parent_name, p.cnt , ch.name child_name, row_number() over (partition by p.parent_id, p.parent_name order by ch.name) rn
from parent p, children ch
where p.parent_id = ch.parent_id
)
start with rn =1
connect by prior rn+1 = rn
group by parent_name,cnt,parent_id