How to get last record from Master-Details tables - sql

Question

I have a table that has 3 columns.
create table myTable
(
ID int Primary key,
Detail_ID int references myTable(ID) null, -- reference to self
Master_Value varchar(50) -- references to master table
)
this table has the follow records:
insert into myTable select 100,null,'aaaa'
insert into myTable select 101,100,'aaaa'
insert into myTable select 102,101,'aaaa'
insert into myTable select 103,102,'aaaa' ---> last record
insert into myTable select 200,null,'bbbb'
insert into myTable select 201,200,'bbbb'
insert into myTable select 202,201,'bbbb' ---> last record
the records is saved In the form of relational with ID and Detail_ID columns.
I need to select the last record each Master_Value column. follow output:
lastRecordID Master_Value Path
202 bbbb 200=>201=>202
103 aaaa 100=>101=>102=>103
tips:
The records are not listed in order in the table.
I can not use the max(ID) keyword. beacuse data is not sorted.(may
be the id column updated manually.)
attempts:
I was able to Prepare follow query and is working well:
with Q as
(
select ID ,Detail_ID, Master_Value , 1 RowOrder, CAST(id as varchar(max)) [Path] from myTable where Detail_ID is null
union all
select R.id,R.Detail_ID , r.Master_Value , (q.RowOrder + 1) RowOrder , (q.[Path]+'=>'+CAST(r.id as varchar(max))) [Path] from myTable R inner join Q ON Q.ID=R.Detail_ID --where r.Dom_ID_RowType=1010
)
select * into #q from Q
select Master_Value, MAX(RowOrder) lastRecord into #temp from #Q group by Master_Value
select
q.ID lastRecordID,
q.Master_Value,
q.[Path]
from #temp t
join #q q on q.RowOrder = t.lastRecord
where
q.Master_Value = t.Master_Value
but I need to simple way (one select) and optimal method.
Can anyone help me?

Answer 1

One method uses a correlated subquery to get the last value (which is how I interpreted your question):
select t.*
from mytable t
where not exists (select 1
from mytable t2
where t2.master_value = t.master_value and
t2.id = t.detail_id
);
This returns rows that are not referred to by another row.
For the path, you need a recursive CTE:
with cte as (
select master_value, id as first_id, id as child_id, convert(varchar(max), id) as path, 1 as lev
from mytable t
where detail_id is null
union all
select cte.master_value, cte.first_id, t.id, concat(path, '->', t.id), lev + 1
from cte join
mytable t
on t.detail_id = cte.child_id and t.master_value = cte.master_value
)
select cte.*
from (select cte.*, max(lev) over (partition by master_value) as max_lev
from cte
) cte
where max_lev = lev
Here is a db<>fiddle.