数据库查询重复记录

简单易理解方法-1

SELECT username, count(*) FROM user GROUP BY username HAVING count(*) > 1

评价：简单有效，效率不高

更多方法看这里，我逐步验证后补充点评

http://www.cnblogs.com/fxgachiever/archive/2010/09/10/1823106.html