Find rows that have the same value on a column in MySQL

Question

In a [member] table, some rows have the same value for the email column.

login_id | email

---------|---------------------

john     | [email protected]

peter    | [email protected]

johnny   | [email protected]

...

Some people used a different login_id but the same email address, no unique constraint was set on this column. Now I need to find these rows and see if they should be removed.

What SQL statement should I use to find these rows? (MySQL 5)

Answer 1

For full rows, you can use this query:

select * from table where email in (

    select email from table

    group by email having count(*) > 1

)

If you want a list of email addresses and how many times they're used, with the most used addresses first. Then use the query given below:

SELECT email,

       count(*) AS c

FROM TABLE

GROUP BY email

HAVING c > 1

ORDER BY c DESC