Delete all Duplicate Rows except for One in MySQL? [duplicate]

Question

How would I delete all duplicate data from a MySQL Table?

For example, with the following data:

SELECT * FROM names;  +----+--------+ | id | name   | +----+--------+ | 1  | google | | 2  | yahoo  | | 3  | msn    | | 4  | google | | 5  | google | | 6  | yahoo  | +----+--------+ 

I would use SELECT DISTINCT name FROM names; if it were a SELECT query.

How would I do this with DELETE to only remove duplicates and keep just one record of each?

Answer 1

Editor warning: This solution is computationally inefficient and may bring down your connection for a large table.

NB - You need to do this first on a test copy of your table!

When I did it, I found that unless I also included AND n1.id <> n2.id, it deleted every row in the table.

1. If you want to keep the row with the lowest id value:

   DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name 

2. If you want to keep the row with the highest id value:

   DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name 

I used this method in MySQL 5.1

Not sure about other versions.

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Update: Since people Googling for removing duplicates end up here
Although the OP's question is about DELETE, please be advised that using INSERT and DISTINCT is much faster. For a database with 8 million rows, the below query took 13 minutes, while using DELETE, it took more than 2 hours and yet didn't complete.

INSERT INTO tempTableName(cellId,attributeId,entityRowId,value)     SELECT DISTINCT cellId,attributeId,entityRowId,value     FROM tableName; 

Answer 2

If you want to keep the row with the lowest id value:

DELETE FROM NAMES  WHERE id NOT IN (SELECT *                      FROM (SELECT MIN(n.id)                             FROM NAMES n                         GROUP BY n.name) x) 

If you want the id value that is the highest:

DELETE FROM NAMES  WHERE id NOT IN (SELECT *                      FROM (SELECT MAX(n.id)                             FROM NAMES n                         GROUP BY n.name) x) 

The subquery in a subquery is necessary for MySQL, or you'll get a 1093 error.