聚合函数

ＡＶＧ:求某一列平均值

ＣＯＵＮＴ:统计总行数

ＳＵＭ：计算列总和

ＭＩＮ:求某一列的最小值

ＭＡＸ：求某一列的最大值

ＡＶＧ:求某一列平均值

SELECT AVG(age) FROM studentinfo;

求某一列值(起一个别名)

SELECT AVG(age) as avgage FROM studentinfo;

SELECT MIN(age) as minage FROM studentinfo;

SELECT MAX(age) as maxage FROM studentinfo;

SELECT SUM(age) FROM studentinfo;

SELECT COUNT(*) FROM studentinfo;

DISTINCT去重，只计算不同的值

SELECT SUM(DISTINCT age) FROM studentinfo;

添加条件，进行聚合函数的计算

SELECT COUNT(gender) FROM studentinfo WHERE gender=1;

SELECT COUNT(gender) FROM studentinfo WHERE gender=0;

分组跟聚合函数的使用，统计每一个分组下有多少人（有多少条记录）

SELECT count(*) as total,age FROM studentinfo GROUP BY age;

统计分组的信息

SELECT count(*), avg(age),min(age),sum(age),gender FROM studentinfo GROUP BY gende；

group by 与　group_concat(列)

group_concat(字段名)可以作为一个输出字段来使用，

表示分组之后，根据分组结果，使用group_concat()来放置每一组的某字段的值的集合

SELECT gender,group_concat(student_name),group_concat(age) FROM studentinfo GROUP BY gender;

group by + with rollup

在最后新增一行，来记录当前列里所有记录的总和

SELECT gender,count(*) FROM studentinfo GROUP BY gender with rollup;

HAVING

HAVING非常类似于WHERE

唯一的差别是WHERE过滤行，而HAVING过滤分组

having 条件表达式：用来分组查询后指定一些条件来输出查询结果

having作用和where类似，但having只能用于group by

SELECT COUNT(),age,adress FROM customers GROUP BY age,adress HAVING COUNT() >= 2;