Many to many cleanup - sql

Question

I have a many to many table that has become a bit of a mess and I'm looking to update the schema to prevent this happening in future. To do that first I need to clean up the existing data to use as the source of a merge script. Not knowing quite how to term this kind of query hitting Google hasn't really helped.
Broken down simply this is what I'm looking at and what I'm aiming to extract.
DECLARE #TestTable
TABLE ([IdA] INT, [IdB] INT);
DECLARE #Expected
TABLE ([KeyId] INT, [MemberId] INT);
INSERT INTO #TestTable
VALUES
(1, 2),
(2, 1),
(1, 3),
(4, 1),
(12, 4),
(5, 6),
(6, 7),
(8, 9),
(11, 10)
INSERT INTO #Expected
VALUES
(1, 1),
(1, 2),
(1, 3),
(1, 4),
(1, 12),
(5, 5),
(5, 6),
(5, 7),
(8, 8),
(8, 9),
(10, 10),
(10, 11)
Lowest Id of a group should be considered the key
Number of members of a group will always be less than 20
Ordering isn't important so long as the resultant dataset is correct
Many thanks.

Answer 1

Here's one way of achieving what you are trying to do:
Here's a SQLFiddle
--Let's copy the rows in as they are to begin with
INSERT INTO
#Expected
SELECT
[IdA]
,[IdB]
FROM
#TestTable
--Lets switch the column values where the higher ID's are in column B
UPDATE #Expected
SET [KeyId] = [MemberId], [MemberId] = [KeyId]
WHERE [KeyId] > [MemberId]
--Now lets set the KEY IDs
UPDATE
E
SET
E.[KeyId] = E1.[KeyId]
FROM
#Expected E
INNER JOIN
#Expected E1
ON
E1.[MemberId] = E.[KeyId]
--and you wanted mappings for the keys to themselves in your result set, so let's add those
INSERT INTO
#Expected
SELECT
[KeyId]
,[KeyId]
FROM
#Expected E
WHERE NOT EXISTS
(SELECT * FROM #Expected WHERE [KeyId] = E.[KeyId] AND [MemberId] = [KeyID])
SELECT DISTINCT
*
FROM
#Expected
Note that I'm selecting DISTINCT at the end because I haven't removed duplicates from the result set.