1.准备
CREATE DATABASE mahaiwuji;
USE mahaiwuji;
CREATE TABLE student (
sid INT (4) PRIMARY KEY,
sname VARCHAR (36),
course VARCHAR (36),
score INT,
grade INT (4)
) ENGINE = INNODB DEFAULT CHARSET = utf8;
INSERT INTO student VALUES (1,'aa','语文',60,1);
INSERT INTO student VALUES (2,'aa','数学',70,1);
INSERT INTO student VALUES (3,'aa','英语',80,1);
INSERT INTO student VALUES (4,'bb','语文',70,1);
INSERT INTO student VALUES (5,'bb','数学',60,1);
INSERT INTO student VALUES (6,'bb','英语',60,1);
INSERT INTO student VALUES (7,'cc','语文',90,2);
INSERT INTO student VALUES (8,'cc','数学',50,2);
INSERT INTO student VALUES (9,'cc','英语',60,2);
INSERT INTO student VALUES (10,'dd','语文',70,2);
INSERT INTO student VALUES (11,'dd','数学',60,2);
INSERT INTO student VALUES (12,'dd','英语',90,2);
s22_1.png
2.实例演示
目的：就是为了统计。
格式：group by 字段的集合。
功能：把表中的记录按照字段分成不同的组。
-- 查询每个人的平均分
SELECT sname,AVG(score) FROM student GROUP BY sname;
s22_2.png
-- 查询每个班的平均分
SELECT grade,AVG(score) FROM student GROUP BY grade;
s22_3.png
-- 查询每个班的每科的平均分
SELECT grade,course,AVG(score) FROM student GROUP BY grade,course;
s24_4.png
