Question

I have a table with a few thousand rows. The table contains two columns, name and email. I have several duplicate rows, for example:

• John Smith | [e-mail]
• John Smith | [e-mail]
• Erica Smith | [e-mail]
• Erica Smith | [e-mail]

What would be the easiest way to delete all duplicate results. For example, such that the table’s content would = SELECT name, DISTINCT(email) FROM table.

Answer 1

You could pretty easily do this by selecting that query into another table, then renaming it to replace the original.

CREATE TABLE `table2` (
  `name` varchar(255), 
  `email` varchar(255), 
  UNIQUE KEY `email` (`email`));
INSERT INTO `table2` SELECT `name`, DISTINCT(`email`) FROM `table`;
RENAME TABLE `table` TO `table1`;
RENAME TABLE `table2` TO `table`;

Note that this CREATE should be adjusted to your actual table format. I added the unique key on the email field as a suggestion on how you would prevent duplicates in the first place.

Alternatively, you could loop over this

DELETE FROM `table` 
WHERE `email` IN (
  SELECT `email` FROM `table` GROUP BY `email` HAVING count(*) > 1
) LIMIT 1

Which would delete one duplicate record per call. The importance of the limit is to not remove both rows for any duplicate