alter table test.student add column indexxx int(14) primary key auto_increment after first; 

DELETE
FROM url2
WHERE ONE IN(SELECT
               link
             FROM (SELECT
                     link
                   FROM url2
                   GROUP BY link
                   HAVING COUNT(link) > 1) a)
    AND id NOT IN(SELECT
                    *
                  FROM (SELECT
                          MIN(rowid)
                        FROM url2
                        GROUP BY link
                        HAVING COUNT(link) > 1) b)