Question

I have a table with customer info. Normally, the PHP checks for duplicates before they new rows are inserted. However, I had to dump a lot of older rows manually and now that they are all in my table, I need to check for duplicates.

Example rows:

id, name, email, phone, fax

I would like to do a mysql query that will show all ID’s with matching emails. I can modify the query later for phone, fax, etc.

I have a feeling I will be using DISTINCT, but I am not quite sure how it’s done.

Answer 1

You can GROUP BY email with HAVING COUNT(*) > 1 to find all duplicate email addresses, then join the resulting duplicate emails with your table to fetch the ids:

SELECT id FROM my_table NATURAL JOIN (
  SELECT email FROM my_table GROUP BY email HAVING COUNT(*) > 1
) t