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

ghz 1years ago ⋅ 3634 views

Question

This question already has answers here :

[Remove duplicate rows in MySQL](/questions/3311903/remove-duplicate-rows-in- mysql) (28 answers)

Closed 3 years ago.

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

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 includedAND 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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