Is it possible for SQL to find records with duplicates?

Question

Can I use a SQL query to find records where one field is identical in both? That is, can I use the following table and return 1,3 (the ids) by comparing the name columns (and ignoring the phone)?

    ID | Name | Phone

    1  | Bob  | 5555555555
    2  | John | 1234567890
    3  | Bob  | 1515151515
    4  | Tim  | 5555555555

Answer 1

To get all names that exist more than once you can execute this statement:

SELECT Name FROM People GROUP BY Name HAVING COUNT(*)>1;

Answer 2

To get the IDs of the duplicates "1,3" concatenated that way use GROUP_CONCAT:

SELECT GROUP_CONCAT( ID SEPARATOR ',' )
FROM Table
GROUP BY Name
HAVING COUNT(*) > 1

Answer 3

Another - not necessarily efficient - way to do this is with a self-join:

SELECT P1.Id, P2.Id
  FROM People P1, People P2
 WHERE P1.Id < P2.Id
   AND P1.Name = P2.Name;

The first condition ensures that you only see the pair (1,3) and not the extraneous pairs (3,1) or the identical rows (1,1), (3,3).