How to remove mySQL duplicate entries

问题

I have a mySQL table with duplicate entries (perhaps in some cases multiple duplicates). I have column called id, which may contain duplicate ids and one called unique id which, as the name suggest contains unique ids. Using the following SQL statement I am able to select the duplicate row

SELECT id,
COUNT(id) AS NumOccurrences
FROM `TABLE 3`
GROUP BY id
HAVING ( COUNT(id) > 1 )

but how do I delete (all but one of) them?

回答1:

DELETE t1 FROM test t1, test t2 WHERE t1.unique_id > t2.unique_id AND t1.id = t2.id;

回答2:

You should add a UNIQUE INDEX on the unique id field. This will prevent more duplicates from sneaking in later.

ALTER IGNORE `TABLE 3` ADD UNIQUE INDEX (`unique_id`);

The IGNORE bit will only keep the first row with the value "unique_id". You should also avoid using spaces in your column or table names. It makes writing SQL error prone as you have to use ticks around names.

来源：https://stackoverflow.com/questions/11621314/how-to-remove-mysql-duplicate-entries