Solved

In MySQL I need to delete some duplicates but leave more than 1

Posted on 2014-10-20
3
138 Views
Last Modified: 2014-10-21
I have a table with a bunch of duplicate records.  This is by design.  A sample may look like:

ID     Date                  location
-----------------------------------------
1      2014-10-20       1
2      2014-10-20       1
3      2014-10-20       1
4      2014-10-20       1
5      2014-10-20       1
6      2014-10-20       1
7      2014-10-20       2
8      2014-10-20       2
9      2014-10-20       2
10    2014-10-20       2
11    2014-10-20       2
12    2014-10-20       2
13      2014-10-21       1
14      2014-10-21       1
15      2014-10-21       1
16      2014-10-21       1
17      2014-10-21       1
18      2014-10-21       1
19      2014-10-21       2
20      2014-10-21       2
21      2014-10-21       2
22      2014-10-21       2
23      2014-10-21       2
24      2014-10-21       2

I want to remove only a certain number of duplicates.  For instance, each location has 6 duplicate dates (6 records for location 1 on 2014-10-20).

I would like to remove just TWO of the duplicate records from each date-location grouping leaving me with 4 records at location 1 on 2014-10-20.  I need to do this for several hundred such groupings.

The ID is primary and unique.

Any help?
0
Comment
Question by:pmascari
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
3 Comments
 
LVL 58

Expert Comment

by:Julian Hansen
ID: 40393841
Can you not  do something like this
CREATE TEMPORARY TABLE deleteids (id INT);
INSERT INTO deleteids (id) SELECT MAX(id) FROM t1 GROUP BY DATE;
DELETE FROM t1 WHERE id IN (SELECT id FROM deleteids);

Open in new window

And then just run the INSERT and DELETE query twice?
0
 
LVL 8

Author Comment

by:pmascari
ID: 40394452
Thanks for the comment.  Your suggestion could work if there were always 6 duplicates.  I've found that these groups can have up to 12 duplicates.  How can I delete a variable number of records, leaving me with groups of 4 for each date/location?
0
 
LVL 58

Accepted Solution

by:
Julian Hansen earned 500 total points
ID: 40394518
Then something like this
SET @ROW := 0, @DATE := '';
DELETE FROM t1 WHERE id IN  (
SELECT id FROM (
  SELECT id, `date`,
      @ROW := IF(@DATE = `date`, @ROW + 1, 1) AS row_number,
      @DATE := `date` AS dummy
  FROM t1
  ORDER BY `date`
) subq
WHERE subq.row_number > 4
);

Open in new window

0

Featured Post

Optimize your web performance

What's in the eBook?
- Full list of reasons for poor performance
- Ultimate measures to speed things up
- Primary web monitoring types
- KPIs you should be monitoring in order to increase your ROI

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This article shows the steps required to install WordPress on Azure. Web Apps, Mobile Apps, API Apps, or Functions, in Azure all these run in an App Service plan. WordPress is no exception and requires an App Service Plan and Database to install
In this series, we will discuss common questions received as a database Solutions Engineer at Percona. In this role, we speak with a wide array of MySQL and MongoDB users responsible for both extremely large and complex environments to smaller singl…
In this brief tutorial Pawel from AdRem Software explains how you can quickly find out which services are running on your network, or what are the IP addresses of servers responsible for each service. Software used is freeware NetCrunch Tools (https…
In this video you will find out how to export Office 365 mailboxes using the built in eDiscovery tool. Bear in mind that although this method might be useful in some cases, using PST files as Office 365 backup is troublesome in a long run (more on t…

624 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question