Extract Unique Pairs from Table Containing Reciprocal Relationships

Question

Consider a SQL Server 2005 database with a bunch of data on a lot of people, a large chunk of whom are married. To track relationships among people, a Relationship table exists that serves to link the ID of one person to the ID of another. A Relationship Type on each Relationship record serves to indicates the type of relationship exists between the two people in question (duh): "is married to", "is son of", "is daughter of", etc.

For married people, there are TWO relationship records in the table--one showing Person-A is married to Person-B and another record showing that Person-B is married to Person-A. For a marriage, it would be considered an error to not have both records, though there are some un-reciprocated records in the DB. (This is an inherited design and I do not have the freedom to change this approach.)

RelID  Person1  Person2  RelationshipType
-----------------------------------------
1      1001     1010     Married           //Reciprocated by RelID 4
2      1002     1011     Married           //Reciprocated by RelID 5
3      1003     1012     Married           //Reciprocated by RelID 6
4      1010     1001     Married           //Reciprocated by RelID 1
5      1011     1002     Married           //Reciprocated by RelID 2
6      1012     1003     Married           //Reciprocated by RelID 3
7      1004     1013     Married           //Not properly reciprocated

What I need to do is extract unique married couples from the table.

I created a function called GetOrderedPair(val1,val2) which takes 2 Person IDs and returns a comma separated string value of both values in sequential order. This way, it always returns the same string value regardless of whether I get, referring to above example data, Person1 and Person2 from record with RelID of 1 or RelID of 4 (a reciprocating pair of records).

I can't help but think that there is a better way that eludes me. Is the use of this function the best approach? What else might you recommend?

Thanks, of course, in advance. :-)

Answer 1

To be honest, I don't entirely understand what your question is. Generically, to fetch a listing of married couples, you would do a simple query like this:

SELECT p1.*, p2.*
FROM   Person AS p1
JOIN   Relationship r ON p1.PersonID = r.Person1 AND r.RelationshipType = 'Married'
JOIN   Person AS p2 ON r.Person2 = p2.PersonID
WHERE  p1.PersonID < p2.PersonID  --This would ensure that you only get each couple "once"

Is this the kind of thing that you're looking for?

Answer 2

Since you've encapsulated GetOrderPair, I have no idea the logic behind it, but I'll at least give you what I do:

select distinct
    case when Person1 > Person2 then Person2 else Person1 end as Person1,
    case when Person1 > Person2 then Person1 else Person2 end as Person2
from
    couples
where
    relationshiptype = 'Married'

Maybe somebody's got a better way of doing this than I do.