Question

Due to a bug in my javascript click handling, multiple Location objects are posted to a JSON array that is sent to the server. I think I know how to fix that bug, but I’d also like to implement a server side database duplicate erase function. However, I’m not sure how to write this query.

The only affected table is laid out as

+----+------------+--------+
| ID | locationID | linkID |
+----+------------+--------+
| 64 |         13 |     14 |
| 65 |         14 |     13 |
| 66 |         14 |     15 |
| 67 |         15 |     14 |
| 68 |         15 |     16 |
| 69 |         16 |     17 |
| 70 |         16 |     14 |
| 71 |         17 |     16 |
| 72 |         17 |     16 |
| 73 |         17 |     16 |
| 74 |         17 |     16 |
| 75 |         17 |     16 |
| 76 |         17 |     16 |
| 77 |         17 |     16 |
+----+------------+--------+

As you can see, I have multiple pairs of (17, 16), while 14 has two pairs of (14, 13) and (14, 15). How can I delete all but one record of any duplicate entries?

Answer 1

Don’t implement post factum correction logic, put a unique index on the fields that need to be unique, that way the database will stop dupe inserts before it’s too late.

If you’re using MySQL 5.1 or higher you can remove dupes and create a unique index in 1 command:

ALTER IGNORE TABLE 'YOURTABLE' 
ADD UNIQUE INDEX somefancynamefortheindex (locationID, linkID)