Hapus duplikat hanya menggunakan kueri MySQL?

Pertimbangkan kasus uji berikut:

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');

Di mana tabel pengujian kami sekarang berisi:

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)

Kemudian kita dapat menggunakan beberapa tabel DELETE sintaks sebagai berikut:

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

... yang akan menghapus entri duplikat, hanya menyisakan url pertama berdasarkan 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)

PERBARUI - Lanjut ke komentar baru di atas:

Jika URL duplikat tidak memiliki format yang sama, Anda mungkin ingin menerapkan REPLACE() berfungsi untuk menghapus www. atau http:// bagian. Misalnya:

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);