修改后的SQL，可应用与RFM分析

 1.分组查询，得到的是按科目分 SELECT  a.sname,a.sno,a.score,  IF( @className = a.project_name,@rank := @rank + 1,@rank :=1 ) AS rank, (@className := a.project_name) project_name           FROM student23   a, ( sel

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