Sort user preferences through their selections and rank - sql

Question

Overview
I'm attempting to create a sorter that allows me to get only possible preferences based on the ranks of users and their preferences
I'm not really sure where to start with this. Below you'll see a SQL Fiddle of a simplified version of what I'm looking at.
http://sqlfiddle.com/#!3/40f0c5/1/0
Initial Code
CREATE TABLE selections
(
id int,
item_id int,
preference int
);
CREATE TABLE ranks
(
id int,
rank int
);
INSERT INTO selections
(id, item_id, preference)
VALUES
(14063, 1, 1),
(14063, 2, 2),
(14063, 3, 3),
(15026, 1, 2),
(15026, 2, 1),
(15026, 3, 3),
(25014, 1, 1),
(25014, 2, 2),
(25014, 3, 3);
INSERT INTO ranks
(id, rank)
VALUES
(14063, 1),
(15026, 2),
(25014, 3);
Expected Outcome
Based on the tables below, if I run the sorter, we should see the results showing the below. Ideally, I would ONLY want to show the item the user got, based on their preference and rank.
14063(1) - item(1)
15026(2) - item(2)
25014(3) - item(3)

Answer 1

I was able to come up with a working solution for you, but it's far from perfect: using a WHILE loop like I'm doing here breaks one of the basic rules of SQL optimization, which is to work with set-based queries as opposed to RBAR. That said, though, I tried coming up with a way to do this with a CTE, with ROW_NUMBER(), and with some NOT EXISTS queries, and failed each time because of the dual nature of the sort. My WHILE loops are pretty unimpressive, so hopefully someone can come along and suggest some improvements for you. There are plenty of people out there whose righteous indignation could probably motivate a criticism or two - hopefully they'll also toss in some ideas or an answer of their own. :)
With that cheerfully self-critical caveat, and wishing you the best of luck on performance, here's a query that will get you the desired resultset:
DECLARE #SortingOutcome TABLE
(
UserID INT,
UserRank INT,
ItemID INT,
ItemPreference INT
)
DECLARE #Looper INT = 1
DECLARE #Ender INT
SELECT #Ender = MAX(Rank) FROM Ranks
WHILE #Looper <= #Ender
BEGIN
INSERT INTO #SortingOutcome
(
UserID,
UserRank,
ItemID,
ItemPreference
)
SELECT TOP 1
r.ID,
rank,
item_id,
preference
FROM
Ranks r
INNER JOIN
Selections s ON
r.id = s.ID
WHERE
r.rank = #Looper AND
NOT EXISTS
(
SELECT 1
FROM #SortingOutcome
WHERE ItemID = s.item_id
)
ORDER BY preference
SET #Looper = #Looper + 1
END
SELECT * FROM #SortingOutcome
SQLFiddle