Get one row of grouped objects - sql

Question

I have a table that contains a Husband to Wife realtion.
The table contains two rows for each realtion(BTW,gender has no meaning.it could be Husband-Husband and Wife-Wife. just saying). meaning, the table might show result of two rows for a "connection":
Wife--Husband and\or Husband--Wife
The table looks like this:
Id1 | Id2 | ConnectiondID | RelatedConnectionId
-----------------------------------------------------
123 | 333 | FF45 | F421
333 | 123 | F421 | FF45
456 | 987 | F333 | F321
987 | 456 | F321 | F333
My expected result is to have only one relation per group:
Id1 | Id2
----------
123 | 333
456 | 987

Answer 1

This is actually very simple assuming you only want couples and your ID values are all unique and numeric, and does not require any self joins, functions or grouping:
declare #t table(Id1 int,Id2 int,ConnectiondID nvarchar(5),RelatedConnectionId nvarchar(5));
insert into #t values(123,333,'FF45','F421'),(333,123,'F421','FF45'),(456,444,'FF46','F422'),(444,456,'F422','FF46'),(789,555,'FF47','F423'),(555,789,'F423','FF47');
select *
from #t
where Id1 < Id2
order by Id1
Output:
+-----+-----+---------------+---------------------+
| Id1 | Id2 | ConnectiondID | RelatedConnectionId |
+-----+-----+---------------+---------------------+
| 123 | 333 | FF45 | F421 |
| 444 | 456 | F422 | FF46 |
| 555 | 789 | F423 | FF47 |
+-----+-----+---------------+---------------------+

Answer 2

If I am understanding your question correctly, you need to perform a self-join on the table e.g. ON t1.id1 = t2.id2 or ON t1.ConnectionId = t2.RelatedConnectionID and obviously this is joining both ways.
To limit this to just one way add a condition on the join predicate such that one of the values is less than or greater than the other; e.g.
DECLARE #tbl table( Id1 smallint PRIMARY KEY, Id2 smallint,ConnectiondID char(5),RelatedConnectionId char(5));
INSERT #tbl(Id1,Id2,ConnectiondID,RelatedConnectionId)
VALUES(123,333,'FF45','F421'),
(333,123,'F421','FF45'),
(456,222,'FF45','F421'),
(222,456,'F421','FF45'),
(789,111,'FF45','F421'),
(111,789,'F421','FF45');
SELECT *
FROM #tbl t1
JOIN #tbl t2 ON t2.Id1 = t1.Id2 AND t2.Id1 > t1.Id1;

Answer 3

For example
DECLARE #T TABLE (id1 int, id2 int,ConnectiondID varchar(5),RelatedConnectionId varchar(5) )
INSERT INTO #T (Id1,Id2,ConnectiondID,RelatedConnectionId)
VALUES
(123 , 333 ,'FF45','F421'),
(333 , 123 , 'F421','FF45'),
(2123 , 2333 ,'2FF45','2F421'),
(2333 , 2123 , '2F421','2FF45'),
(3 , 2 , 'AAAA','BBB'),
(2 , 3 , 'BBB','AAAA')
SELECT
a.*
FROM
#t a
WHERE
CASE
WHEN ConnectiondID > RelatedConnectionId
THEN RelatedConnectionId
ELSE NULL
END IS NULL