Highway of Life Highway of Life - 10 days ago 6
SQL Question

Delete all Duplicate Rows except for One in MySQL?


Possible Duplicate:

Remove duplicate rows in MySQL




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

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.

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;
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