Iterating through a social graph in a SQL database - sql

Question

I store simple social-graph information like so:
People ( PersonId bigint, Name nvarchar )
Relationships ( From bigint, To bigint, Title nvarchar )
So the data looks something like this:
People
1, John Smith
2, Joan Smith
3, Jack Smith
Relationships
1, 2, Spouse
1, 3, Parent
2, 3, Parent
Note that the titles of relationships are normalized: so there is no "husband" and "wife", only "spouse", which also avoids needing to create two separate relationships that form the same link, the same applies with "Parent" instead of "Son" or "Daughter".
The question is how you can iterate through an entire connected-graph (i.e. only return a single family), and, for example, find siblings without needing to create an explicit Sibling relationship entry. The nodes don't necessarily need to be returned in any particular order. I might also want to only return nodes that are at most N degrees away from a given start node.
I know you can do recursive SQL SELECT statements with some new tricks in recent SQL language versions, but this isn't necessarily a recursive operation because these relationships can express a cyclic non-directional graph (think if "Friend" was added as a relationship). How would you do that in SQL?

Answer 1

Very cool problem. While it's a social network graph, it is still a hierarchical problem, even though the hierarchy can logistically turn into a web of interconnections. In MSSQL you still want to use a WITH clause to do a recursive query, the only difference is that due to the multiple interconnections you need to ensure unique results, either with DISTINCT or by using an IN clause in the WHERE condition.
This works:
DECLARE #PersonID bigint;
SET #PersonID = 1;
WITH RecurseRelations (PersonID, OriginalPersonID)
AS
(
SELECT PersonID, PersonId OriginalPersonID
FROM People
UNION ALL
SELECT ToPersonID, RR.OriginalPersonID
FROM Relationships R
INNER JOIN
RecurseRelations RR
ON
R.FromPersonID = RR.PersonID
)
SELECT PersonId, Name
FROM People
WHERE PersonId IN
(
SELECT PersonID
FROM RecurseRelations
WHERE OriginalPersonID = #PersonID
)
Here's some test data with more relations than you had originally and a whole other family to make sure it's not picking up more than intended.
create table People ( PersonId bigint, Name nvarchar(200) );
create table Relationships ( FromPersonID bigint, ToPersonID bigint, Title nvarchar(200) );
insert into People values (1, 'John Smith');
insert into People values (2, 'Joan Smith');
insert into People values (3, 'Jack Smith');
insert into People values (4, 'Joey Smith');
insert into People values (9, 'Jaime Smith');
insert into People values (5, 'Edward Jones');
insert into People values (6, 'Emma Jones');
insert into People values (7, 'Eva Jones');
insert into People values (8, 'Eve Jones');
insert into Relationships values (1, 2, 'Spouse');
insert into Relationships values (1, 3, 'Parent');
insert into Relationships values (2, 3, 'Parent');
insert into Relationships values (3, 4, 'Child');
insert into Relationships values (2, 4, 'Child');
insert into Relationships values (4, 9, 'Child');
insert into Relationships values (5, 6, 'Spouse');
insert into Relationships values (5, 7, 'Parent');
insert into Relationships values (6, 7, 'Parent');
insert into Relationships values (5, 8, 'Child');