SQL Find most rows that match between two tables - sql

Question

I am using SQL Server 2012 I have two tables like the following
Table1 and Table 2 both have many groups, indicated by the group column. The name of the group may match in both tables, but it may not. What is important is finding the group on Table 2 that has the most members that match members in a group on Table1.
I first tried doing this with a vlookup, but the problem is vlookup pulls the first entry in the Group column that has a match, not the group with the most matches. Below vlookup would pull BBB, but the correct result is CCC.
Ties may occur. There might be more than one group on Table2 that match Table1 with the same number of members thus the best thing may be to count the number of matches, but there are thousands of groups so it's not ideal to sort and sift through a column with counts. I need something like a case statement where if there is a MAX(match) then Table1 would show the group name with MAX(Match) in the derived column BestMatch. It'd be most ideal if the column could display all the groups on table 2 that have MAX(Match which may be one or more. Perhaps it could be comma separated.
If not if the column could just say tie and I could look for the tie, it'd be ideal if this is the best option, when the word tie appears it repeats besides every member that matches so I will know to look for groups that matching which accounts and how many that matched.

Answer 1

We really could do with some expected output to help clarify the question.
If I understand you correctly however, this query will get you close to the results you require:
;with cte as
( SELECT t1a.[group] AS Group1
, t2a.[Group] AS Group2
, RANK() OVER(PARTITION BY t1a.[group]
ORDER BY COUNT(t2a.[Group]) DESC) AS MatchRank
FROM Table1 t1a
JOIN Table2 t2a
ON t1a.member = t2a.member
GROUP BY t1a.[group], t2a.[GRoup])
SELECT *
FROM cte
WHERE MatchRank=1
The query doesn't identify ties, but it will display any tied results...
If you are a newbie to common table expressions(the ;with statement) there is a useful description here.

Answer 2

select *
from Table1 t1
outer apply
(
select top 1 t2.[Group]
from Table2 t2
where t2.Member = t1.Member
group by t2.[Group]
order by count(*) desc
) m

Answer 3

It may not be the most elegant solution but I think it could do the work:
select *
from
(select t1.[group] as t1group, t1.member, t2.[group] as t2group
from Table1 t1 inner join Table2 t2 on t1.member = t2.member)a
where member = (select max(t1.member)
from Table1 t1 inner join Table2 t2 on t1.member = t2.member)
In case of 2 rows from Table2 matching the maximum members in Table1, both results would be displayed
PS: an example of your desired results would have been helpful

Answer 4

Count member matches per group pair and rank them so the group pairs with the highest match count get rank #1. Once you found these, you can select the related records from table1 and table2.
select t1.grp, t1.member, t2.grp
from t1
join
(
select
t1.grp as grp1,
t2.grp as grp2,
rank() over (order by count(*) desc) as rnk
from t1
join t2 on t2.member = t1.member
group by t1.grp, t2.grp
) grps on grps.rnk = 1 and grps.grp1 = t1.grp
left join t2 on t2.grp = grps.grp2 and t2.member = t1.member
order by t1.grp, t1.member, t2.grp;
This gives you ties in separate rows, e.g. for AAA having four different members (123,456,789,555) with two matches both in CCC and DDD:
grp1 member grp2
AAA 123 CCC
AAA 123 DDD
AAA 456 CCC
AAA 789
AAA 555 DDD
If you want one row per grp1 and member with all matching grp2 in a string then you need some clumsy STUFF trick in SQL Server as far as I am aware. Look up "GROUP_CONCAT in SQL Server" to find the technique needed.