1，排序，根据总分 增加序号
表结构：
sql：
SET @row=0;
SELECT b.*, @row:=@row+1 rownum FROM (
select *,SUM(a.achievement) as achievementAll from cp_cj as a
where 1=1 GROUP BY a.studentNum order by SUM(a.achievement) DESC limit 0,30
) as b
结果：
2，排序，根据总分 增加序号，并且按照总分相同的并列
sql：
select c.* ,ifnull((
select count(*) from
(
select *,SUM(a.achievement) as achievementAll from cp_cj as a
where 1=1 GROUP BY a.studentNum order by SUM(a.achievement) DESC
) as b
where c.achievementAll < b.achievementAll
),0)+1 as rownum from
(
select *,SUM(a.achievement) as achievementAll from cp_cj as a
where 1=1 GROUP BY a.studentNum order by SUM(a.achievement) DESC limit 0,30
) as c
结果：
