SQL - select rows that have the same value in two columns

Question

The solution to the topic is evading me.

I have a table looking like (beyond other fields that have nothing to do with my question):

NAME,CARDNUMBER,MEMBERTY

Answer 1

What's the most efficient way of doing this?

I believe a JOIN will be more efficient than EXISTS

SELECT t1.* FROM myTable t1
JOIN (
    SELECT cardnumber, membertype
    FROM myTable
    GROUP BY cardnumber, membertype
    HAVING COUNT(*) > 1
) t2 ON t1.cardnumber = t2.cardnumber AND t1.membertype = t2.membertype

Query plan: http://www.sqlfiddle.com/#!2/0abe3/1