分数匹配等级

将sname，degree，rank三表组合匹配成一张新表

grade表
scores表
student表

SELECT sname,degree,rank FROM students 
INNER JOIN scores ON (students.`sno` = scores.`sno`)
INNER JOIN grade ON 
(scores.`degree` >= grade.`low` AND scores.`degree` <= grade.`upp`)
ORDER BY degree DESC