Finding the “deepest” child in a hierarchical query
-
26-06-2021 - |
Вопрос
I need some help querying hierarchical data. Here is the single, simple, table, where parent_id
references id
and may be null for root entries.
create table edition (
id NUMBER(20),
parent_id NUMBER(20)
);
For each record in the table I need to find the deepest child having maximum id. If a record has no children, then its own id should be returned. I tried by myself but failed using START WITH A.id = B.id
where A and B are subqueries, looks like Oracle doesn't allow such joins.
Here is the sample data:
id parent_id
----------------------
1 NULL
2 1
3 1
4 1
5 4
6 5
7 5
and a sample result
id result
----------------------
1 7
2 2
3 3
4 7
5 7
6 6
7 7
Решение
I believe you want to try
create table tq84_edition (
id number primary key,
parent_id number references tq84_edition
);
insert into tq84_edition values ( 1, null);
insert into tq84_edition values ( 2, 1);
insert into tq84_edition values ( 3, 1);
insert into tq84_edition values ( 4, 1);
insert into tq84_edition values ( 5, 4);
insert into tq84_edition values ( 6, 5);
insert into tq84_edition values ( 7, 5);
with x (root, id, parent_id, lvl) as (
select id root,
id,
parent_id,
1 lvl
from tq84_edition
UNION ALL
select x.root root,
tq84_edition.id,
tq84_edition.parent_id,
x.lvl + 1 lvl
from x,
tq84_edition
where x.id = tq84_edition.parent_id
)
select root, max(id) keep (dense_rank last order by lvl, id)
from x
group by root;
Другие советы
Another approach I can think of, it's easier to port on other RDBMS: http://www.sqlfiddle.com/#!4/da0a3/19
with parent(root_node, child_of, parent_id, depth) as
(
select id, id, parent_id, 1
from edition
union all
select p.root_node, e.id, e.parent_id, p.depth + 1
from edition e
join parent p on p.child_of = e.parent_id
)
select root_node, max(child_of)
from parent
where (root_node,depth) in
(select root_node,max(depth) from parent group by root_node)
group by root_node
order by root_node
Output:
| ROOT_NODE | MAX(CHILD_OF) |
-----------------------------
| 1 | 7 |
| 2 | 2 |
| 3 | 3 |
| 4 | 7 |
| 5 | 7 |
| 6 | 6 |
| 7 | 7 |
Now, I'm loving Oracle (and http://sqlfiddle.com too), it's very concise. Now I know what's the use of MIN and MAX in KEEP DENSE_RANK. Whereas before I don't see any utility on explicitly specifying MIN/MAX in KEEP DENSE_RANK. Now I know it has a utility, if there's some ties on depth, you can see who is first and last in ties by using MIN and MAX.
e.g. http://www.sqlfiddle.com/#!4/da0a3/24
with parent(root_node, child_of, parent_id, depth) as
(
select id, id, parent_id, 1
from edition
union all
select p.root_node, e.id, e.parent_id, p.depth + 1
from edition e
join parent p on p.child_of = e.parent_id
)
select root_node,
min(child_of) keep(dense_rank last order by depth) as first_in_deepest,
max(child_of) keep(dense_rank last order by depth) as last_in_deepest
from parent
group by root_node;
| ROOT_NODE | FIRST_IN_DEEPEST | LAST_IN_DEEPEST |
--------------------------------------------------
| 1 | 6 | 7 |
| 2 | 2 | 2 |
| 3 | 3 | 3 |
| 4 | 6 | 7 |
| 5 | 6 | 7 |
| 6 | 6 | 6 |
| 7 | 7 | 7 |