SQL find sets with common members (relational division)

Question

I have separate sets of "classes" and "groups", each of which has been assigned one or more tags. I would like to find, for each group, the subset of classes that contains the same (or more) tags for each group.

Some sample data:

declare @Groups table
(
    GroupID int,
    TagID int
)

insert @Groups
values (1,1),(1,2),(1,3),
    (2,1),(2,2),
    (3,1),(3,2),(3,3),(3,4)

declare @Classes table
(
    ClassID int,
    TagID int
)

insert @Classes
values (1,1),(1,2),
    (2,1),(2,2),
    (3,1),(3,2),(3,3)

select * from @Groups
select * from @Classes

And output:

GroupID TagID
1       1
1       2
1       3
2       1
2       2
3       1
3       2
3       3
3       4

ClassID TagID
1       1
1       2
2       1
2       2
3       1
3       2
3       3

An example result set would look like this:

declare @Results table
(
    GroupID int,
    ClassID int
)

insert @Results
values (1,3),(2,1),(2,2),(2,3),(3,null)

select * from @Results

Results output:

GroupID ClassID
1       3
2       1
2       2
2       3
3       NULL

I understand this is a relational division type problem, involving having and count. These posts describe what I want to do, but I can't figure out how to apply the examples to the particular case above:

• http://weblogs.sqlteam.com/peterl/archive/2010/07/02/Proper-Relational-Division-With-Sets.aspx
• SQL Join only if all records have a match
• How can I structure a query to give me only the rows that match ALL values in a CSV list of IDs in T-SQL
• SQL: select sets containing exactly given members
• An sql query to find users with common tags in a defined set

Answer 1

I think this should also work

select distinct g.GroupID, c.ClassID
from @Groups g
    left join @Classes c on g.TagID = c.TagID
where not exists (
    select *
    from @Groups g2
    where g2.GroupID = g.GroupID
        and g2.TagID not in (
            select TagID
            from @Classes c2
            where c2.ClassID = c.ClassID
        )
    ) or c.ClassID is null

Answer 2

You can join the tables together, and demand that all tags from the group are found in the class:

select  g.GroupID
,       c.ClassID
from    @Groups g
join    @Classes c
on      c.TagID = g.TagID
group by
        g.GroupID
,       c.ClassID
having  count(c.TagID) =
        (
        select  count(*)
        from    @Groups g2
        where   g2.GroupID = g.GroupID
        )

This does not list groups without a matching class, and I can't think of a simple way to do so.

Example at SQL Fiddle.