Find Duplicate Customers
To find rows where at least three out of four of those columns match you can use.

SELECT  D1.ID, D2.ID
FROM   DATA D1
       JOIN DATA D2
         ON D1.SSN = D2.SSN
            AND D1.ID &gt; D2.ID
            AND 2 &lt;= CASE
                       WHEN D1.FNAME = D2.FNAME THEN 1
                       ELSE 0
                     END +
                     CASE
                       WHEN D1.LNAME = D2.LNAME THEN 1
                       ELSE 0
                     END  +
                     CASE
                       WHEN D1.DOB = D2.DOB THEN 1
                       ELSE 0
                     END
UNION ALL
SELECT D1.ID, D2.ID
FROM   DATA D1
       JOIN DATA D2
         ON D1.DOB = D2.DOB
         AND  D1.FNAME = D2.FNAME
         AND  D1.LNAME = D2.LNAME
         AND D1.SSN &lt;&gt; D2.SSN
         AND D1.ID &gt; D2.ID


The top branch gets all rows where the SSN are the same and at least 2 out of the three other columns are the same. The join on SSN is likely to be pretty selective in itself.

That just leaves one other possible three column combination left which is dealt with by the second branch.

Both branches of the UNION ALL have an equi join so it should perform better than a join with some complicated OR condition.