基本语法
SELECT column-list FROM table_name
WHERE [ conditions ]
GROUP BY column1,column2
HAVING [ conditions ]
ORDER BY column1,column2
注意顺序 → WHERE - GROUP BY - HAVING - ORDER BY
SELECT classno,COUNT(studentname) FROM student GROUP BY classno; 按照classno字段，分组汇总学生数量
SELECT classno,MIN(age),AVG(age) FROM student GROUP BY classno; 查看每个班级年龄最小值/均值
INSERT INTO company3 VALUES(7,‘pual‘,2000);
INSERT INTO company3 VALUES(8,‘allen‘,3000);
INSERT INTO company3 VALUES(9,‘teddy‘,20000);
SELECT name,SUM(salary) FROM company3 GROUP BY name; 多插入一些重复名称的数据，汇总每个员工的总薪水
分组+排序
SELECT classno,COUNT(studentname) FROM student GROUP BY classno ORDER BY classno; 按照classno排序
SELECT name,SUM(salary) FROM company3 GROUP BY name ORDER BY SUM(salary); 按照总薪水排序
过滤分组，WHERE
SELECT classno,COUNT(studentname) FROM student
WHERE classno > 2
GROUP BY classno; WHERE作用与分组前，这里先筛选classno>2的数据，再分组
注意，这里如果书写：WHERE COUNT(studentname) > 1 就会报错
过滤分组，HAVING
SELECT classno,COUNT(studentname) FROM student
GROUP BY classno
HAVING COUNT(studentname) > 1; HAVING主要用于过滤分组，且是在分组后进行过滤
所以一般对于分组的条件过滤，都用HAVING
