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Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
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Removing duplicates from a MySQL table

Surely there's got to be an easy way to remove duplicate records from a MySQL table? With a well designed table that has a unique primary key, you can simply get rid of a few duplicates, but if there are a lot of 'em, it's time consuming. You could write some sort of complex delete based on a select. You could write an application to do it (it wouldn't be more that a few lines of Perl, for example) but surely there's got to be an easy way!

A neat trick for which I take no credit ... thanks to a Paul Swarthout who gave it as an answer [here] on Database Journal.

1. Create a Unique Index on the column that should be unique but isn't, using the ignore keyword to suppress error messages ... it will zap the fault records instead.

2. Optionally (if you decide you didn't really want the index ;-) ) take the index away again.

Let's see an example of that:



OOOOOOoops!

alter ignore table demo add unique index dwarf_name (name(20))



Features of each Dwarf from Pub Quiz Help - it's not the sort of thing I know much about myself.

If you want to try this for yourself, the data may be found [here].

Aside - before you remove duplicates, be very careful to define what you actually want to do. Do you want to remove records with duplicates in a particular column (which is what this example does) or do you want to remove rows where the complete row (except for the unique ID field?) is identical?


(written 2010-02-22, updated 2010-03-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
S161 - Data Access and Security in MySQL
  [192] Current MySQL and PHP paths and upgrades - (2005-01-28)
  [193] The wrong MySQL - (2005-01-29)
  [401] What is an SQL injection attack? - (2005-08-02)
  [535] MySQL permissions and privileges - (2005-12-20)
  [647] Checking for MySQL errors - (2006-03-15)
  [947] What is an SQL injection attack? - (2006-11-27)
  [1131] MySQL - Password security (authentication protocol) - (2007-04-02)
  [2204] Images in a database? How big is a database? (MySQL) - (2009-05-28)
  [2263] Mysqldump fails as a cron job - a work around - (2009-06-30)
  [3270] SQL - Data v Metadata, and the various stages of data selection - (2011-04-29)
  [4390] Checking MySQL database backups have worked (not failed) - (2015-01-10)
  [4406] Fixing damaged MySQL tables - Error 1712 and Error 2013 - (2015-01-25)
  [4481] Extracting data from backups to restore selected rows from MySQL tables - (2015-05-01)
  [4491] Web Server Admin - some of those things that happen, and solutions - (2015-05-10)
  [4493] Forgotten / lost MySQL root password - (2015-05-16)

S157 - More MySQL commands
  [158] MySQL - LEFT JOIN and RIGHT JOIN, INNER JOIN and OUTER JOIN - (2004-12-20)
  [159] MySQL - Optimising Selects - (2004-12-21)
  [279] Getting a list of unique values from a MySQL column - (2005-04-14)
  [449] Matching in MySQL - (2005-09-24)
  [494] MySQL - a score of things to remember - (2005-11-12)
  [502] SELECT in MySQL - choosing the rows you want - (2005-11-22)
  [513] MySQL - JOIN or WHERE to link tables correctly? - (2005-12-01)
  [515] MySQL - an FAQ - (2005-12-03)
  [517] An occasional chance, and reducing data to manageable levels - (2005-12-04)
  [567] Combining similar rows from a MySQL database select - (2006-01-17)
  [572] Giving the researcher power over database analysis - (2006-01-22)
  [581] Saving a MySQL query results to your local disc for Excel - (2006-01-29)
  [591] Key facts - SQL and MySQL - (2006-02-04)
  [673] Helicopter views and tartans - (2006-04-06)
  [1213] MySQL - the order of clauses and the order of actions - (2007-06-01)
  [1235] Outputting numbers as words - MySQL with Perl or PHP - (2007-06-17)
  [1331] MySQL joins revisited - (2007-09-03)
  [1574] Joining MySQL tables revisited - finding nonmatching records, etc - (2008-03-15)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2110] MySQL - looking for records in one table that do NOT correspond to records in another table - (2009-03-31)
  [2259] Grouping rows for a summary report - MySQL and PHP - (2009-06-27)
  [2448] MySQL - efficiency and other topics - (2009-10-10)
  [2643] Relating tables with joins in MySQL - (2010-02-21)
  [2644] Counting rows in joined MySQL tables - (2010-02-22)
  [2645] Optimising and caching your MySQL enquiries - (2010-02-22)
  [3061] Databases - why data is split into separate tables, and how to join them - (2010-11-20)


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Removing duplicates from a MySQL table
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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