MySQL---分组查询

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--每个年级有多少人
SELECT COUNT (StudentName), SGrade as 年级
FROM S2
GROUP BY SGrade

--总分
SELECT SUM(Sorce) as 总分
FROM S2
GROUP BY SGrade

--每科平均分
SELECT SGrade,AVG(Sorce) AS 课程平均分
From S2
GROUP BY SGrade

--按顺序
SELECT SGrade,AVG(Sorce) AS 课程平均分
From S2
GROUP BY SGrade
ORDER BY AVG(Sorce) DESC --倒叙

--多列分组
SELECT COUNT(StudentName),sGrade as 年级,Sex as 性别
FROM S2
GROUP BY SGrade,Sex
ORDER BY SGrade