Mysql query to find all rows that have the same values as another row

Question

My database contains rows that generally look like:

PersonItem
__________
id
personId
itemId

╔════╦══════════╦════════╗
║ ID ║ PERSONID ║ ITEMID ║
╠════╬═══         


        

Answer 1

You need to find examples where the pair of personid/itemid appear more than once. In MySQL you can do this with a where clause and subquery:

select t.*
from t
where exists (select 1
              from t t2
              group by personid, itemid
              having count(*) > 1 and
                     t2.personid = t.personid and t2.itemid = t.itemid
             )

The above is standard SQL. MySQL also supports the multi-column in statement. So this can be written as:

select t.*
from t
where (t.personid, t.itemid) in (select personid, itemid
                                 from t
                                 group by personid, itemid
                                 having count(*) > 1
                                )

And alternative that I like, based on Eugene's answer but more efficient, is:

SELECT t.personid, t.ItemId, GROUP_CONCAT(t.ID)
FROM t
GROUP BY t.personid, t.ItemId
HAVING COUNT(*) > 1;

It does away with any joins, if you don't mind getting the ids as a list rather than as separate rows.