How to delete duplicate records in mysql database?

Question

What's the best way to delete duplicate records in a mysql database using rails or mysql queries?

Answer 1

What you can do is copy the distinct records into a new table by:

 select distinct * into NewTable from MyTable

Answer 2

This question is regarding SQL but i hope it helps.

Answer 3

New to SQL :-) This is a classic question - often asked in interviews:-) I don't know whether it'll work in MYSQL but it works in most databases -

> create table t(
>     a char(2),
>     b char(2),
>     c smallint )

> select a,b,c,count(*) from t
> group by a,b,c
> having count(*) > 1
a  b  c
-- -- ------ -----------
(0 rows affected)

> insert into t values ("aa","bb",1)
(1 row affected)

> insert into t values ("aa","bb",1)
(1 row affected)

> insert into t values ("aa","bc",1)
(1 row affected)

> select a,b,c,count(*) from t group by a,b,c having count(*) > 1
a  b  c 
-- -- ------ -----------
aa bb      1           2
(1 row affected)

Answer 4

I had to do this recently on Oracle, but the steps would have been the same on MySQL. It was a lot of data, at least compared to what I'm used to working with, so my process to de-dup was comparatively heavyweight. I'm including it here in case someone else comes along with a similar problem.

My duplicate records had different IDs, different updated_at times, possibly different updated_by IDs, but all other columns the same. I wanted to keep the most recently updated of any duplicate set.

I used a combination of Rails logic and SQL to get it done.

Step one: run a rake script to identify the IDs of the duplicate records, using model logic. IDs go in a text file.

Step two: create a temporary table with one column, the IDs to delete, loaded from the text file.

Step three: create another temporary table with all the records I'm going to delete (just in case!).

CREATE TABLE temp_duplicate_models 
  AS (SELECT * FROM models 
  WHERE id IN (SELECT * FROM temp_duplicate_ids));

Step four: actual deleting.

DELETE FROM models WHERE id IN (SELECT * FROM temp_duplicate_ids);

Answer 5

well, if it's a small table, from rails console you can do

class ActiveRecord::Base
  def non_id_attributes
    atts = self.attributes
    atts.delete('id')
    atts
  end
end

duplicate_groups = YourClass.find(:all).group_by { |element| element.non_id_attributes }.select{ |gr| gr.last.size > 1 }
redundant_elements = duplicate_groups.map { |group| group.last - [group.last.first] }.flatten
redundant_elements.each(&:destroy)

Answer 6

You can use:

http://lenniedevilliers.blogspot.com/2008/10/weekly-code-find-duplicates-in-sql.html

to get the duplicates and then just delete them via Ruby code or SQL code (I would do it in SQL code but thats up to you :-)

Answer 7

Here's another idea in no particular language:

rs = `select a, b, count(*) as c from entries group by 1, 2 having c > 1`
rs.each do |a, b, c|
  `delete from entries where a=#{a} and b=#{b} limit #{c - 1}`
end

---

Edit:

Kudos to Olaf for that "having" hint :)

Answer 8

If your table has a PK (or you can easily give it one), you can specify any number of columns in the table to be equal (to qualify is as a duplicate) with the following query (may be a bit messy looking but it works):

DELETE FROM table WHERE pk_id IN(
   SELECT DISTINCT t3.pk_id FROM (
       SELECT t1.* FROM table AS t1 INNER JOIN (
           SELECT col1, col2, col3, col4, COUNT(*) FROM table
           GROUP BY col1, col2, col3, col4 HAVING COUNT(*)>1) AS t2
       ON t1.col1 = t2.col1 AND t1.col2 = t2.col2 AND t1.col3 = t2.col3 AND
       t1.col4 = t2.col4)
   AS t3, (
       SELECT t1.* FROM table AS t1 INNER JOIN (
           SELECT col1, col2, col3, col4, COUNT(*) FROM table
           GROUP BY col1, col2, col3, col4 HAVING COUNT(*)>1) AS t2
       ON t1.col1 = t2.col1 AND t1.col2 = t2.col2 AND t1.col3 = t2.col3 AND
       t1.col4 = t2.col4)
   AS t4
   WHERE t3.col1 = t4.col1 AND t3.pk_id > t4.pk_id

)

This will leave the first record entered into the database, deleting the 'newest' duplicates. If you want to keep the last record, switch the > to <.

Answer 9

In MySql when I put something like

delete from A where IDA in (select IDA from A )

mySql said something like "you can't use the same table in the select part of the delete operation."

I've just have to delete some duplicate records, and I have succeeded with a .php program like that

<?php
...
$res = hacer_sql("SELECT MIN(IDESTUDIANTE) as IDTODELETE 
FROM `estudiante` group by `LASTNAME`,`FIRSTNAME`,`CI`,`PHONE`
HAVING COUNT(*) > 1 )");
while ( $reg = mysql_fetch_assoc($res) ) {
   hacer_sql("delete from estudiante where IDESTUDIANTE = {$reg['IDTODELETE']}");
}
?>

Answer 10

Hi,

I am using alter table

ALTER IGNORE TABLE jos_city ADD UNIQUE INDEX(city);