考虑以下测试用例:
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');
我们的测试表现在包含:
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)
然后我们就可以使用多表了DELETE语法如下:
DELETE t2
FROM mytb t1
JOIN mytb t2 ON (t2.url_addr = t1.url_addr AND t2.url_id > t1.url_id);
...这将删除重复的条目,只留下第一个基于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- 进一步补充上述新评论:
如果重复的 URL 不具有相同的格式,您可能需要应用REPLACE()删除函数www.
or http://
部分。例如:
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);