Question

I have a table with columns for ID, firstname, lastname, address, email and so on.

Is there any way to delete duplicate email addresses from the TABLE?

Additional information (from comments):

If there are two rows with the same email address one would have a normal firstname and lastname but the other would have ‘Instant’ in the firstname. Therefore I can distinguish between them. I just want to delete the one with first name ‘instant’.

Note, some records where the firstname='Instant' will have just 1 email address. I don’t want to delete just one unique email address, so I can’t just delete everything where firstname='Instant'.

Please help me out.

Answer 1

I don’t know if this will work in MYSQL (I haven’t used it)… but you should be able to do something like the following snippets.

I’d suggest you run them in order to get a feel for if the right data is being selected. If it does work, then you probably want to create a constraint on the column.

Get all of the duplicate e-mail addresses:

SELECT 
    EMAILADDRESS, COUNT(1)
FROM
    TABLE
GROUP BY EMAILADDRESS
HAVING COUNT(1) > 1

Then determine the ID from that gives:

SELECT
    ID
FROM 
    TABLE
WHERE 
    EMAILADDRESS IN (
        SELECT 
            EMAILADDRESS
        FROM
            TABLE
        GROUP BY EMAILADDRESS
        HAVING COUNT(1) > 1
    )

Then finally, delete the rows, based on the above and other constraints:

DELETE 
FROM 
    TABLE
WHERE
    ID IN (
        SELECT
            ID
        FROM 
            TABLE
        WHERE 
            EMAILADDRESS IN (
                SELECT 
                    EMAILADDRESS
                FROM
                    TABLE
                GROUP BY EMAILADDRESS
                HAVING COUNT(1) > 1
            )
    )  
    AND FIRSTNAME = 'Instant'