Question

I have created a pretty messy table that I need to clean up. This is not an easy task.

Situation:

• Messy table is called tbl_users.
• It contains information about a user. It has a unique ID.
• The most unique colomn is phone_number which identifies a user.

For varoius reasons I have the same user with the same phone number more than once in this table.

You can see when it is created in a colomn called created_date.

What i need:

• I need to find the FIRST created instance of each user and then I
  need to find
• all the ID’s of the second and even maybe third instance of the user,
  if
  such exists.

I need the ID’s because there’s data from other tables where I need to change the user_id to first instance.

How do I proceed with this challenge?

Answer 1

select t.*, (t.created_date = tf.minDate) as IsFirst
from tbl_users t
inner join (
    select phone_number, min(created_date) as minDate
    from tbl_users
    group by phone_number
) tf on t.phone_number = tf.phone_number