Get nth level on self-referencing table - sql

Question

I have a self-referencing table which has at max 5 levels
groupid | parentid | detail
--------- | --------- | ---------
Group A | Highest | nope
Group B | Group A | i need this
Highest | NULL | nope
Group C | Group B | nope
Group D | Group C | nope
I have a transaction table which lookups to the groupid on the table above to retrieve the detail value where groupid = Group B. The values of the groupid on the transaction table is only between Group B to D and will never go any higher.
txnid | groupid | desired | desired
--------- | --------- | --------- | ---------
1 | Group D | Group B | i need this
2 | Group B | Group B | i need this
3 | Group C | Group B | i need this
4 | Group B | Group B | i need this
How should my T-SQL script be like to attain the desired column? I can left join to the self referencing table multiple times to get until group B it's not consistent on how many time I need to join back.
Greatly appreciate any thoughts!

Answer 1

Still not clear to me how do you know which is the GROUP B, I suppose it's the record where the parent of it parent is null.
create table org(groupid char(1), parentid char(1), details varchar(20));
insert into org values
('a', null, 'nope'),('b', 'a', 'I need this'),('c', 'b', 'nope'),('d', 'c', 'nope'),('e', 'd', 'nope');
create table trans(id int, groupid char(1));
insert into trans values
(1, 'b'),(2, 'c'),(3, 'c'),(4, 'd'),(5, 'e');
GO
10 rows affected
with all_levels as
(
select ob.groupid groupid_b, oc.groupid groupid_c,
od.groupid groupid_d, oe.groupid groupid_e,
ob.details
from org ob
inner join org oc
on oc.parentid = ob.groupid
inner join org od
on od.parentid = oc.groupid
inner join org oe
on oe.parentid = od.groupid
where ob.parentid is not null
) select * from all_levels;
GO
groupid_b | groupid_c | groupid_d | groupid_e | details
:-------- | :-------- | :-------- | :-------- | :----------
b | c | d | e | I need this
--= build a 4 levels row
with all_levels as
(
select ob.groupid groupid_b, oc.groupid groupid_c,
od.groupid groupid_d, oe.groupid groupid_e,
ob.details
from org ob
inner join org oc
on oc.parentid = ob.groupid
inner join org od
on od.parentid = oc.groupid
inner join org oe
on oe.parentid = od.groupid
where ob.parentid is not null
)
--= no matter what groupid returns b group details
, only_b as
(
select groupid_b as groupid, groupid_b, details from all_levels
union all
select groupid_c as groupid, groupid_b, details from all_levels
union all
select groupid_d as groupid, groupid_b, details from all_levels
union all
select groupid_e as groupid, groupid_b, details from all_levels
)
--= join with transactions table
select id, t.groupid, groupid_b, ob.details
from trans t
inner join only_b ob
on ob.groupid = t.groupid;
GO
id | groupid | groupid_b | details
-: | :------ | :-------- | :----------
1 | b | b | I need this
2 | c | b | I need this
3 | c | b | I need this
4 | d | b | I need this
5 | e | b | I need this
dbfiddle here
You can deal with a recursive function too, but I don't believe it can be better on terms of performance.
create function findDetails(#groupid char(1))
returns varchar(100)
as
begin
declare #parentid char(1) = '1';
declare #next_parentid char(1) = '1';
declare #details varchar(100) = '';
while #next_parentid is not null
begin
select #details = org.details, #parentid = org.parentid, #next_parentid = op.parentid
from org
inner join org op
on op.groupid = org.parentid
where org.groupid = #groupid
set #groupid = #parentid;
end
return #details;
end
GO
✓
select id, groupid, dbo.findDetails(groupid) as details_b
from trans;
GO
id | groupid | details_b
-: | :------ | :----------
1 | b | I need this
2 | c | I need this
3 | c | I need this
4 | d | I need this
5 | e | I need this
dbfiddle here