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一、统计函数的使用

#count统计函数的使用#统计一个班级共有多少学生SELECT COUNT(*)FROM student;#统计语文成绩大于60的学生SELECT COUNT(*) FROM student WHERE chinese>60#统计总分大于200分的SELECT COUNT(*) FROM student WHERE (chinese+english+math)>200#count(*)与count(列)区别#count(*)返回满足条件记录的行数#count(列)返回某列有多少个，但会排除为null的情况CREATE TABLE db13(    id INT);INSERT INTO db13 VALUES(1);INSERT INTO db13 VALUES(2);INSERT INTO db13 VALUES(NULL);SELECT COUNT(*) FROM db13SELECT COUNT(id) FROM db13 #sum函数的使用#统计一个班的语文总成绩SELECT SUM(chinese)AS '语文' FROM student#统计各科总成绩SELECT SUM(chinese)AS '语文' ,SUM(english)AS '英语',SUM(math)AS '数学' FROM student#统计所有成绩总和SELECT SUM(chinese+english+math) FROM student#统计数学总成绩平均分SELECT SUM(math)/COUNT(math) AS'数学平均分' FROM student#avg函数的使用#求一个班级语文平均分SELECT AVG(chinese) FROM student;#求一个班级总分平均分SELECT AVG(chinese+english+math) FROM student;#max和min的使用#统计班级最高分与最低分SELECT MAX(chinese+english+math),MIN(chinese+english+math) FROM student;#统计班级语文最高分与最低分SELECT MAX(chinese),MIN(chinese) FROM student