Easy way of deleting duplicates in mysql

[brown2005]

hi i have a table and i want to be able to delete duplicates say from table a field a..

 

is there an easy way of doing this?

[marcus]

Well if you're using a script to have it be inserted into the database you could do.

 

$sql = "SELECT * FROM `table` WHERE `field1` ='$field1'";
$res = mysql_query($sql) or die(mysql_error());
if(mysql_num_rows($res) > 0){
//dont insert
}else {
//insert query
}

[utexas_pjm]

Define "easy"...

 

DELETE 
FROM table_a 
WHERE id in 
(
                       SELECT id
                       FROM table_a 
                       GROUP BY column_a 
                       HAVING COUNT(*) > 1
)

 

Best,

 

Patrick

[brown2005]

SQL query: 

 

DELETE FROM tablea WHERE email IN (

 

SELECT email

FROM tablea

GROUP BY email

HAVING COUNT( * ) >1

)

 

MySQL said: 

 

#1064 - You have an error in your SQL syntax.  Check the manual that corresponds to your MySQL server version for the right syntax to use near 'SELECT email

FROM tablea

GROUP BY email

HAVING COUNT(*) >

 

i get that mate

[utexas_pjm]

Hmmm... Can you post your version of MySQL?  I'll try to do this old school with joins, give me a few mins.

 

Patrick

[brown2005]

MySQL client version: 4.1.11

 

that bit?

[utexas_pjm]

Yes, that's what I need.  Furthermore let me explain something before I wipe out your data.  The SQL I gave you will delete all duplicates in the literal sense.  That is, any email which appears more than once will be wiped out.  So if the email [email protected] appeared twice, after running the query all records containing that email address would be deleted.  I don't think, looking at your query, that is your intention.  It seems that you want to delete all subsequent duplicate email addresses, preserving the original.  Let me know if this sounds right.

 

Patrick

[brown2005]

yes, i want to keep one email record, and delete any records with that email.. exactly..

[utexas_pjm]

hmmm.... try this, preferably on a dummy table first:

 

DELETE tablea 
FROM tablea AS a1, tablea AS a2 
WHERE a1.email = a2.email 
AND a1.id > a2.id 

 

This query assumes that you have an id column that is sequential.

 

Patrick

[brown2005]

thanks very much, works perfectly