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MYSQL remove duplicates

Posted on 2014-10-09
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Last Modified: 2014-10-26
Hi All I know this is really simple but im a bit of a dunse with MYSQL so wanted a bit of advice,  I have a table "Activefeed" which has a coumn in it "email".  I would like to remove all of the rows which have a duplicate email addresss,  i.e if there are 2 rows with the same email address delete on of the rows and so on.  I thought this would of been easy but there is a lot of different options which creating and removing rows etc,   all I want to end up with is my table "activefeed" to effectively be cleaned and only have 1 row per email address and all duplicates deleted.  

Apologies but in  your answer can you treat me like an idiot ;)
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Question by:ncomper
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Expert Comment

by:chaau
ID: 40372011
There is an easy way to remove the duplicates and to prevent them to re-appear in future. It is by using a UNIQUE index on the column. There is a MySQL syntax for the index creation that allows to remove any duplicate rows:
    ALTER IGNORE TABLE activefeed ADD UNIQUE INDEX idx_email (email);

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By using IGNORE keyword you instruct MySQL to remove duplicates. However, there is no control on what record is chosen by MySQL. For example, there are emails attached to different accounts. You may know that email is really for account B rather than A, because account A has not used your system for a few years now. MySQL does not know this, and it can delete the email for account B instead. If you have ideas what records to keep, share them with us and we can help you
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by:ncomper
ID: 40372386
Error Code: 1062. Duplicate entry 'geoff.hall@allihashhill.co.uk' for key 'idx_email'
 
Hi i get this error when I run it,  thank you,  Although if possible I would like to know a way that it can run using delete / distinct commands if possible (have also seen code where you create a new temp table
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by:chaau
ID: 40372395
It is very strange. According to this document (http://dev.mysql.com/doc/refman/5.1/en/alter-table.html) the IGNORE keyword must automatically remove duplicates. What version of MySQL are you using?
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Stampel earned 500 total points
ID: 40372495
Maybe you can try to create a new empty table with the same structure as your original table, then just add unique constrait on your email field. After this, just run :
insert into table_dest (select * from table_src);

the duplicate rows will not be allowed !
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