How to loop through table using while loop and create another table with values needed - sql

Question

I have two tables: MainTable and MyTable. MyTable has unique ControlNo and ID. I need to add very first EffDate from MainTable to MyTablebased on ID and ControlNo.
For that I need to look at PreviousID column, then see if that PreviousID is in ID column and so on.
Desired output should look like this:
The below is an example with dummy data of getting proper EffDate by supplying an ID value. It works, but how can I loop through the whole MainTable, retrieve ID's and EffDate into separate table, then join that table to MyTable?
-- function returns PreviousID based on ID
CREATE FUNCTION [dbo].[GetPriorQuoteID](#ID varchar(50))
RETURNS varchar(50)
AS
BEGIN
DECLARE #RetVal varchar(50)
SET #RetVal = NULL
SELECT TOP 1 #RetVal = MainTable.PreviousID
FROM MainTable
WHERE MainTable.ID = #ID
RETURN #RetVal
END
-- create sample table
IF OBJECT_ID('MainTable') IS NOT NULL DROP TABLE MainTable;
select 3333 as ControlNo, 'QuoteID3' as ID, 'QuoteID2' as PreviousID, '2020-08-25' as EffDate
into MainTable
union all select 2222 as COntrolNo, 'QuoteID2', 'QuoteID1', '2019-08-25'
union all select 1111 as COntrolNo, 'QuoteID1', NULL, '2018-08-25'
union all select 7777 as COntrolNo, 'QuoteID6', 'QuoteID5', '2020-02-10'
union all select 6666 as COntrolNo, 'QuoteID5', NULL, '2019-02-10'
select * from MainTable
DECLARE #PriorQuote varchar(50)
DECLARE #RetVal VARCHAR(50) = ''
DECLARE #ControlNo INT
DECLARE #ID varchar(50) = 'QuoteID3'
SELECT TOP 1 #ControlNo = MainTable.ControlNo FROM MainTable WHERE MainTable.ID = #ID
Set #PriorQuote = #ID
SELECT TOP 1 #PriorQuote = MainTable.ID FROM MainTable WHERE MainTable.ControlNo = #ControlNo
WHILE dbo.GetPriorQuoteID(#PriorQuote) IS NOT NULL AND dbo.GetPriorQuoteID(#PriorQuote)<> #PriorQuote
BEGIN
SET #PriorQuote = dbo.GetPriorQuoteID(#PriorQuote)
END
SELECT TOP 1 #RetVal = CONVERT(VARCHAR(10), MainTable.EffDate, 101)
FROM MainTable
WHERE MainTable.ID = #PriorQuote
SELECT #RetVal
-- clean up
drop table MainTable
drop function GetPriorQuoteID
UPDATE: Adding dummy data tables
-- create sample table #MainTable
IF OBJECT_ID('tempdb..#MainTable') IS NOT NULL DROP TABLE #MainTable;
create table #MainTable (ControlNo int, ID varchar(50), PreviousID varchar(50), EffDate date)
insert into #MainTable values
(3333,'QuoteID3','QuoteID2', '2020-08-25'),
(2222,'QuoteID2','QuoteID1', '2019-08-25'),
(1111,'QuoteID1',NULL, '2018-08-25'),
(7777,'QuoteID6','QuoteID5', '2020-02-10'),
(6666,'QuoteID5',NULL, '2019-02-10')
--select * from #MainTable
-- create sample table #MyTable
IF OBJECT_ID('tempdb..#MyTable') IS NOT NULL DROP TABLE #MyTable;
create table #MyTable (ControlNo int, ID varchar(50), EffDate date)
insert into #MyTable values
(3333,'QuoteID3',NULL),
(7777,'QuoteID6',NULL)
--select * from #MyTable

Answer 1

You can use a recursive query to traverse the hierarchy.
I would start by joining the original table with the main table, which restricts the paths to just the rows we are interested in. Then, you can recurse towards the parent. Finally, we need to filter on the top parent per path: top() and row_number() come handy for this.
Consider:
with cte as (
select t.controlno, t.id, m.previousid, m.effdate, 1 lvl
from #maintable m
inner join #mytable t on t.controlno = m.controlno and t.id = m.id
union all
select c.controlno, c.id, m.previousid, m.effdate, c.lvl + 1
from cte c
inner join #maintable m on m.id = c.previousid
)
select top(1) with ties controlno, id, effdate
from cte
order by row_number() over(partition by controlno, id order by lvl desc)
Demo on DB Fiddle:
controlno | id | effdate
--------: | :------- | :---------
3333 | QuoteID3 | 2018-08-25
7777 | QuoteID6 | 2019-02-10

Answer 2

using CTE like below you can get the desired results.
See live demo
Learn more about recursive CTEs here
; with cte as
(
select EffDate, ControlNo, ID, Level=1 from MainTable
where PreviousID is NULL
union all
select C.EffDate, M.ControlNo, M.ID, Level=Level+1 from MainTable AS M
join cte as C on C.ID=M.PreviousID
)
select MyTable.*,cte.EffDate from cte join MyTable on Mytable.ID=cte.ID

Answer 3

You can use a recursive CTE for this:
WITH cte
AS
(
SELECT m.ID,m.PreviousID
FROM MainTable m
JOIN MainTable m2
ON m.previousID = m2.ID
WHERE m2.previousID IS NULL
UNION ALL
SELECT m2.ID,cte.previousID
FROM cte
JOIN MainTable m2
ON m2.previousID = cte.ID
)
SELECT *
FROM cte;

Answer 4

Here is a working example of the CTE approach with the table provided
;with recur_cte(ControlNo, ID, PreviousID, EffDate, HLevel) as (
select mt.ControlNo, cast(null as varchar(100)), mt.PreviousID, mt.EffDate, 1
from MainTable mt
where not exists(select 1
from MainTable mt_in
where mt.ID=mt_in.PreviousID)
union all
select rc.ControlNo, rc.ID, mt.PreviousID, mt.EffDate, rc.HLevel+1
from recur_cte rc
join MainTable mt on rc.PreviousID=mt.ID and rc.EffDate>mt.EffDate)
select * from recur_cte;
Results
ControlNo ID PreviousID EffDate HLevel
3333 NULL QuoteID2 2020-08-25 1
7777 NULL QuoteID5 2020-02-10 1
7777 NULL NULL 2019-02-10 2
3333 NULL QuoteID1 2019-08-25 2
3333 NULL NULL 2018-08-25 3