How to select highest level from 2 different trees in same command - sql

Question

I have a dataset that has the following appearence.
A 1
A 2
A 3
B 1
B 2
B 3
B 4
Which is a result from using the following command
select
connect_by_root id as root,
level lvl
from
dbset
start with id in ('A','B')
connect by nocycle child = prior parent)
I want the result
A 3
B 4
That is, I want to extract the topmost root in each of the trees. Ideally I would like this to be done within the same command I have, but I am too much of a novice within the area to know how to.

Answer 1

Use ROW_NUMBER:
SELECT root, lvl
FROM
(
SELECT
connect_by_root id AS root,
level lvl,
row_number() over (PARTITION BY connect_by_root id ORDER BY level DESC) rn
FROM dbset
START WITH id IN ('A','B')
CONNECT BY NOCYCLE child = prior parent
) t
WHERE rn = 1

Answer 2

Just use group by :
create table t23 ( dt varchar(10) ,id1 int );
insert all
into t23 values('A',1)
into t23 values('A',2)
into t23 values('A',3)
into t23 values('B',1)
into t23 values('B',2)
into t23 values('B',3)
into t23 values('B',4)
select * from dual;
select * from t23;
select dt , max(id1)id1 from t23 group by dt;

Answer 3

The connect_by_isleaf function might be what you're after, e.g.:
WITH dbset AS (SELECT 'A' ID, 1 CHILD, NULL PARENT FROM dual UNION ALL
SELECT 'A' ID, 2 CHILD, 1 PARENT FROM dual UNION ALL
SELECT 'A' ID, 3 CHILD, 2 PARENT FROM dual UNION ALL
SELECT 'B' ID, 1 CHILD, NULL PARENT FROM dual UNION ALL
SELECT 'B' ID, 2 CHILD, 1 PARENT FROM dual UNION ALL
SELECT 'B' ID, 3 CHILD, 2 PARENT FROM dual UNION ALL
SELECT 'B' ID, 4 CHILD, 3 PARENT FROM dual)
SELECT connect_by_root ID AS root,
LEVEL lvl,
CONNECT_by_isleaf,
CHILD, PARENT
FROM dbset
WHERE CONNECT_by_isleaf = 1
START WITH ID IN ('A', 'B') AND PARENT IS NULL
CONNECT BY NOCYCLE PRIOR CHILD = PARENT
AND ID = PRIOR ID
ORDER BY root;
ROOT LVL CONNECT_BY_ISLEAF CHILD PARENT
---- ---------- ----------------- ---------- ----------
A 3 1 3 2
B 4 1 4 3

Answer 4

This is basically the same as Tim Biegeleisen's answer, just completed with test data, result, sql fiddle and a correction about the partitioning condition.
Note: I am happily delete this answer as soon as Tim's answer is corrected and completed.
Test data:
create table dbset (id varchar(1), child varchar(1), parent varchar(1) );
insert into dbset values('A','A','A');
insert into dbset values('X','A','X');
insert into dbset values('Z','X','Z');
insert into dbset values('B','6','6');
insert into dbset values('G','6','7');
insert into dbset values('H','7','8');
insert into dbset values('I','8','9');
Query:
SELECT root, lvl
FROM
(
SELECT
connect_by_root id AS root,
level lvl,
row_number() over (PARTITION BY connect_by_root id ORDER BY level DESC) rn
FROM dbset
START WITH id IN ('A','B')
CONNECT BY NOCYCLE child = prior parent
) t
WHERE rn = 1;
Result:
| ROOT | LVL |
|------|-----|
| A | 3 |
| B | 4 |
Sql fiddle: http://sqlfiddle.com/#!4/37c70/8

Answer 5

Use ranking function row_number() to get the top most record for each root.
select * from
(
select *,
row_number() over (partition by id order by level desc) Seq
from table
) t
where Seq = 1