SQL Determine the teams having same members - sql

Question

I have an interesting problem.
I've some teams with a team leader stored in one table and the members of teams are stored in child table. I want to determine the teams that have same members.
TEAMS
TEAM_ID LEADER_ID
1 1
2 1
3 2
4 2
MEMBERS
TEAM_ID MEMBER_ID
1 2
1 3
1 4
2 3
2 4
2 5
3 1
3 3
3 4
4 5
4 6
4 7
I was able to write this query to determine the formations and now I am clueless how to proceed.
SELECT
TEAM_ID,
(
SELECT
CONVERT (VARCHAR, MEMBER_ID) + ', '
FROM
(
SELECT
TEAM_ID,
LEADER_ID AS MEMBER_ID
FROM
TEAMS
UNION ALL
SELECT
TEAM_ID,
MEMBER_ID
FROM
MEMBERS
) FORMATIONS
WHERE
TEAM_ID = MT.TEAM_ID
ORDER BY
MEMBER_ID FOR XML PATH ('')
) AS MEMBERS
FROM
TEAMS MT
As it is clear that team id 1 and 3 are same, how can the lowest ID of the duplicate teams can be obtained.
i.e. the query should return the list of TEAM_IDs that are smallest per duplicate group (and only if they are duplicate)
In this scenario id 1 should be returned.
http://sqlfiddle.com/#!18/c845a/5

Answer 1

There are worse ways to approach this than stuffing the members into a string and comparing them. So, I'll follow the route you have started.
All you need to do is to combine the members from the two tables, and then use that for the logic:
with m as (
select team_id, member_id
from members
union -- on purpose to remove duplicates
select team_id, leader_id
from teams
)
select *
from (select team_id, members, count(*) over (partition by members) as num_teams
from (select t.team_id,
stuff( (select concat(',', m.member_id)
from m
where m.team_id = t.team_id
order by m.member_id
for xml path ('')
), 1, 1, ''
) as members
from teams t
) t
) t
where num_teams > 1
order by members;
Here is your SQL Fiddle.
Note that string comparison works fine for this case, which is an exact match of members. For superset relationships is doesn't work so well.

Answer 2

Try below query, it uses CTE to get grouped teams wth its members listed as comma separated list.
Later it is used with group by to determine lowest ID of teams with same members. To ensure thhat there will be only duplicated teams, I used having clause.
;with cte as (
select team_id,
(select cast(member_id as varchar(5)) + ',' from #members innerMembers
where team_id = m.team_id
and not exists(select 1 from #TEAMS
where leader_id = innerMembers.member_id)
order by member_id
for xml path('')) members
from #members m
group by team_id
)
select min(team_id), members from cte
group by members
having count(*) > 1

Answer 3

Using pure SQL.
The main idea is that two sets A and B being equal is defined by A being a subset of B and B being a subset of A.
And we can check whether B is a subset of A by getting the members of B which are in A, counting them, and checking whether this equals the count in A.
As this is a somewhat complicated step I simply did it by cross applying a subquery filtered to teams A and B. There may be a more elegant way.
WITH MembersAll AS
(
SELECT Team_Id, Member_Id FROM Members
UNION
-- Consider leaders as members.
SELECT Team_Id, Leader_Id AS Member_Id FROM Teams
),
-- Teams and any teams which are a subset of that team:
TeamSubsetTeam AS (
SELECT
ThisTeam.Team_Id,
OtherTeam.Team_Id AS SubsetTeam_Id
FROM Teams AS ThisTeam
CROSS JOIN Teams AS OtherTeam -- Considering all pairs of teams.
CROSS APPLY (
-- Get the members in both teams,
-- left join so that we have all members from a given team
-- and all of the members in the other team that are in the given team
-- then filter on the counts of these being the same.
SELECT
COUNT(MembersThisTeam.Member_Id) AS MemberCountThisTeam,
COUNT(MembersOtherTeamInThisTeam.Member_Id) AS MemberCountOtherTeamInThisTeam
FROM MembersAll AS MembersThisTeam
LEFT JOIN MembersAll AS MembersOtherTeamInThisTeam
ON MembersThisTeam.Member_Id = MembersOtherTeamInThisTeam.Member_Id
AND MembersOtherTeamInThisTeam.Team_Id = OtherTeam.Team_Id
WHERE MembersThisTeam.Team_Id = ThisTeam.Team_Id
) MemberCounts
WHERE MemberCounts.MemberCountThisTeam = MemberCounts.MemberCountOtherTeamInThisTeam
),
-- Teams and any teams which are equivalent to that team (including itself):
TeamEquivalentTeam AS (
-- From set theory, team A is equivalent to team B if
-- team A is a subset of team B and
-- team B is a subset of team A.
SELECT
Team_Id,
SubsetTeam_Id AS EquivalentTeamId
FROM TeamSubsetTeam
WHERE Team_Id IN (
SELECT SubsetTeam_Id FROM TeamSubsetTeam AS SubsetTeamSubsetTeam
WHERE SubsetTeamSubsetTeam.Team_Id = TeamSubsetTeam.SubsetTeam_Id
)
)
-- The specified post-processing step.
-- Doesn't seem particularly useful but you can do whatever you like
-- now you have the information in TeamEquivalentTeam.
SELECT DISTINCT MIN(EquivalentTeamId) AS FirstEquivalentTeam
FROM TeamEquivalentTeam
GROUP BY Team_Id
Returns:
FirstEquivalentTeam
1
2
4