How to re write while loop using cte - sql-server

Question

I have two tables, one with Events, the other with episodes.
An Episode has a start date and end date, the event has a single date.
Both Episodes and Events have one of six Types.
Currently I'm using some fuzzy logic to run an update script on the Events table to set it's ID field to the matching Episode. It does this by checking for the Event date between the Episode start and end, both having the same Type, as well as some other links like same User etc.
Since the Events can sit outside of the Episode, or have a different Type, what I do is loop through a sequence of expanding date ranges (StartDate-1, -2 etc) and also cycle through each Type looking for a match.
I've been reading that while loops aren't very efficient, so was wondering if there was a way to rewrite this nested loop into a CTE function.
I'm using SQL Server 2012.
Event List is just a temp table that has all the possible Types with an order to loop through.
My loop currently is:
WHILE #CurrBefore <= #Before and #CurrentAfter <= #After
BEGIN
SET #Row = 0
WHILE #Row <= #MaxRow
BEGIN
UPDATE P
SET P.ID = E.ID
FROM Event P
OUTER APPLY (SELECT TOP 1 E.Id, E.Type
FROM Episode E
WHERE E.User = P.User AND
E.Type = CASE WHEN #Row=0 THEN P.Event ELSE (SELECT Event FROM #EventList WHERE RN = #Row) END AND
P.Date BETWEEN E.StartDate-#CurrentBefore AND E.EndDate+#CurrentAfter
ORDER BY P.Date) E
WHERE P.ID = 0
INCREMENT #ROW CODE
END
INCREMENT #BEFORE/AFTER CODE
END
Sample Data:
IF OBJECT_ID('tempdb..#EventList') IS NOT NULL
BEGIN
DROP TABLE #EventList
CREATE TABLE #EventList(Event Varchar(50), RN INT);
INSERT INTO #EventList SELECT 'A', 1
INSERT INTO #EventList SELECT 'B', 2
INSERT INTO #EventList SELECT 'C', 3
INSERT INTO #EventList SELECT 'D', 4
INSERT INTO #EventList SELECT 'E', 5
INSERT INTO #EventList SELECT 'F', 6
END
CREATE TABLE dbo.Episode ([ID] INT, [Start] DateTime, [End] DateTime, [Type] varchar(1), [User] INT)
INSERT INTO [dbo].Episode ([ID], [Start], [End], [Type],[User])
VALUES
(1, '2018-07-01 10:00', '2018-07-02 14:00', 'A',10),
(2, '2018-07-05 6:00', '2018-07-06 13:00', 'A',11),
(3, '2018-07-03 9:00', '2018-07-04 8:00', 'B',10),
(4, '2018-07-02 15:00', '2018-07-03 7:00', 'B',12),
(5, '2018-07-01 1:00', '2018-07-02 8:00', 'C',13),
(6, '2018-07-01 6:00', '2018-07-01 8:00', 'D',11)
CREATE TABLE dbo.Event ([ID] INT, [Date] DateTime, [Type] varchar(1), [User] INT)
INSERT INTO [dbo].Event ([ID], [Date], [Type],[User])
VALUES
(0, '2018-07-01 12:00', 'A',10),
(0, '2018-07-05 15:00', 'A',11),
(0, '2018-07-03 13:00', 'C',10),
(0, '2018-07-10 9:00', 'B',12),
(0, '2018-07-01 5:00', 'C',10),
(0, '2018-07-01 10:00', 'D',11)
Expected result, Event now looks like this:
1 2018-07-01 12:00:00.000 A 10
2 2018-07-05 15:00:00.000 A 11
3 2018-07-03 13:00:00.000 C 10
0 2018-07-10 09:00:00.000 B 12
1 2018-07-01 05:00:00.000 C 10
6 2018-07-01 10:00:00.000 D 11

Answer 1

I don't know, if I fully got the logic, but this might help to get you running:
USE master;
GO
CREATE DATABASE TestDB
GO
USE TestDB;
GO
CREATE TABLE dbo.Episode ([ID] INT, [Start] DateTime, [End] DateTime, [Type] varchar(1), [User] INT)
INSERT INTO [dbo].Episode ([ID], [Start], [End], [Type],[User])
VALUES
(1, '2018-07-01 10:00', '2018-07-02 14:00', 'A',10),
(2, '2018-07-05 6:00', '2018-07-06 13:00', 'A',11),
(3, '2018-07-03 9:00', '2018-07-04 8:00', 'B',10),
(4, '2018-07-02 15:00', '2018-07-03 7:00', 'B',12),
(5, '2018-07-01 1:00', '2018-07-02 8:00', 'C',13),
(6, '2018-07-01 6:00', '2018-07-01 8:00', 'D',11)
CREATE TABLE dbo.[Event] ([ID] INT, [Date] DateTime, [Type] varchar(1), [User] INT)
INSERT INTO [dbo].[Event] ([ID], [Date], [Type],[User])
VALUES
(0, '2018-07-01 12:00', 'A',10),
(0, '2018-07-05 15:00', 'A',11),
(0, '2018-07-03 13:00', 'C',10),
(0, '2018-07-10 9:00', 'B',12),
(0, '2018-07-01 5:00', 'C',10),
(0, '2018-07-01 10:00', 'D',11)
GO
CREATE TABLE #EventList(Event Varchar(50), RN INT);
INSERT INTO #EventList VALUES ('A', 1),('B', 2),('C', 3),('D', 4),('E', 5),('F', 6);
WITH mathingEpisodes AS
(
SELECT ev.ID AS evID
,ev.[Date] AS evDate
,ev.[Type] AS evType
,ev.[User] AS evUser
,e1.RN AS evRN
,ep.ID AS epID
,ep.[Type] AS epType
,e2.RN AS epRN
FROM [Event] ev
LEFT JOIN Episode ep ON ev.[User]=ep.[User] AND ev.[Date] >= ep.[Start] AND ev.[Date] < ep.[End]
LEFT JOIN #EventList e1 ON ev.[Type]=e1.[Event]
LEFT JOIN #EventList e2 ON ep.[Type]=e2.[Event]
)
SELECT COALESCE(epID,Closest.ID) AS FittingEpisodeID
,me.evDate
,evType
,evUser
FROM mathingEpisodes me
OUTER APPLY(SELECT TOP 1 *
FROM Episode ep
CROSS APPLY(SELECT ABS(DATEDIFF(SECOND,me.evDate,ep.[Start])) AS DiffToStart
,ABS(DATEDIFF(SECOND,me.evDate,ep.[End])) AS DiffToEnd) Diffs
CROSS APPLY(SELECT CASE WHEN DiffToStart<DiffToEnd THEN DiffToStart ELSE DiffToEnd END AS Smaller) Diffs2
WHERE ep.[User] = me.evUser
AND me.epID IS NULL
ORDER BY Diffs2.Smaller
) Closest
ORDER BY evDate;
GO
USE master;
GO
DROP DATABASE TestDB;
GO
DROP TABLE #EventList
GO
The result
1 2018-01-07 05:00:00.000 C 10
6 2018-01-07 10:00:00.000 D 11
1 2018-01-07 12:00:00.000 A 10
3 2018-03-07 13:00:00.000 C 10
2 2018-05-07 15:00:00.000 A 11
4 2018-10-07 09:00:00.000 B 12
Some explanation
In the first cte I try to find fitting episodes (same user and date within range).
The second cte will compute the closest Episode for the same user in all cases, where the first cte did not succeed.
The only difference for this sample is the event for userId=12. My logic will bind this to the closest episode of this user (ID=4), while your expected output shows a zero in this place.
Anyway, my solution is fully set-based, therefore faster than a loop, and should be rather close to your needs. Try to adapt it...
UPDATE Some more thoughts...
I did not get the ghist of your #EventList... I bound the results into the set (you can make it visible by using SELECT * instead of the explicit column list. But this is - assumably - not what you meant...