select @rownum:=@rownum+1 rownum, seq_id from ( select @rownum:=0, seq_id from seq order by seq_id desc limit 20) t;
参考：
[url]http://www.cnblogs.com/advocate/archive/2012/03/02/2376900.html [/url]
http://blog.csdn.net/ACMAIN_CHM/article/details/4095531
Oracle
rownum
SQL Server
row_number()