Select users with more than one instance in database

Question

I have a table with a userID field and an itemID field. I would like to select all of those users that have two or more instances where itemID is the same (that is, if for example there are 3 records where userID = 1 and itemID = 7 then I would like those results, but not if there's just one instance). I need to get all users (not just results for a certain userID). Can anybody suggest how I could do this? Thanks.

Answer 1

You can do this using aggregation and a having clause. If you just want the users:

select distinct userID
from t
group by userId, itemID
having count(*) >= 2;

This is an interesting query because it is one of the very rare situations where group by and select distinct are used together. If you wanted the userId / itemId pairs, then you would use select userId, itemId , without the distinct .

Answer 2

You just need to use group by and having. The having clause is like where except that it also works on aggregations. So it's something like select userID, itemID, count(*) from mytable group by userID, itemID having count(*) > 1 .