MySql数据库查询——使用聚合函数查询

COUNT()用来统计记录的总条数，一般与GROUP BY关键字一起使用

SELECT COUNT(*) FROM `grade`;

SELECT COUNT(*) AS total FROM `grade`;

SELECT stuName,COUNT(*) AS total FROM grade GROUP BY stuName;

SUM（）函数是求和函数 与GROUP BY关键字一起使用

SELECT stuName,SUM(grade) FROM grade WHERE stuName='小王';

SELECT stuName,SUM(grade) FROM grade GROUP BY stuName;

AVG()函数是求平均值的函数与GROUP BY关键字一起使用

SELECT stuName,AVG(grade) FROM grade WHERE stuName='小王';

SELECT stuName,AVG(grade) FROM grade GROUP BY stuName;

MAX()函数是求最大值的函数与GROPE BY关键字一起使用

SELECT stuName,course,MAX(grade) FROM grade WHERE stuName='小王';

SELECT stuName,course,MAX(grade) FROM grade GROUP BY stuName;

MIN()函数是求最大值的函数与GROPE BY关键字一起使用

SELECT stuName,course,MIN(grade) FROM grade WHERE stuName='小王';

SELECT stuName,course,MIN(grade) FROM grade GROUP BY stuName;