Remove duplicates using only a MySQL query?

Question

I have a table with the following columns:

URL_ID    
URL_ADDR    
URL_Time

I want to remove duplicates on the URL_ADDR column using a MySQL query.

Is it possible to do such a thing without using any programming?

Answer 1

Consider the following test case:

CREATE TABLE mytb (url_id int, url_addr varchar(100));

INSERT INTO mytb VALUES (1, 'www.google.com');
INSERT INTO mytb VALUES (2, 'www.microsoft.com');
INSERT INTO mytb VALUES (3, 'www.apple.com');
INSERT INTO mytb VALUES (4, 'www.google.com');
INSERT INTO mytb VALUES (5, 'www.cnn.com');
INSERT INTO mytb VALUES (6, 'www.apple.com');

Where our test table now contains:

SELECT * FROM mytb;
+--------+-------------------+
| url_id | url_addr          |
+--------+-------------------+
|      1 | www.google.com    |
|      2 | www.microsoft.com |
|      3 | www.apple.com     |
|      4 | www.google.com    |
|      5 | www.cnn.com       |
|      6 | www.apple.com     |
+--------+-------------------+
5 rows in set (0.00 sec)

Then we can use the multiple-table DELETE syntax as follows:

DELETE t2
FROM   mytb t1
JOIN   mytb t2 ON (t2.url_addr = t1.url_addr AND t2.url_id > t1.url_id);

... which will delete duplicate entries, leaving only the first url based on url_id:

SELECT * FROM mytb;
+--------+-------------------+
| url_id | url_addr          |
+--------+-------------------+
|      1 | www.google.com    |
|      2 | www.microsoft.com |
|      3 | www.apple.com     |
|      5 | www.cnn.com       |
+--------+-------------------+
3 rows in set (0.00 sec)

---

UPDATE - Further to new comments above:

If the duplicate URLs will not have the same format, you may want to apply the REPLACE() function to remove www. or http:// parts. For example:

DELETE t2
FROM   mytb t1
JOIN   mytb t2 ON (REPLACE(t2.url_addr, 'www.', '') = 
                   REPLACE(t1.url_addr, 'www.', '') AND 
                   t2.url_id > t1.url_id);

Answer 2

You may want to try the method mentioned at http://labs.creativecommons.org/2010/01/12/removing-duplicate-rows-in-mysql/.

ALTER IGNORE TABLE your_table ADD UNIQUE INDEX `tmp_index` (URL_ADDR);

Answer 3

This will leave the ones with the highest URL_ID for a particular URL_ADDR

DELETE FROM table
WHERE URL_ID NOT IN 
    (SELECT ID FROM 
       (SELECT MAX(URL_ID) AS ID 
        FROM table 
        WHERE URL_ID IS NOT NULL
        GROUP BY URL_ADDR ) X)   /*Sounds like you would need to GROUP BY a 
                                   calculated form - e.g. using REPLACE to 
                                  strip out www see Daniel's answer*/

(The derived table 'X' is to avoid the error "You can't specify target table 'tablename' for update in FROM clause")