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Jun 01, 2016 pm 01:06 PM

汇总数据

有时对数据表的操作不是表中数据本身，而是表中数据的汇总，例如某一列数据的平均值，最大值，最小值等。而对于这些常用的数据汇总处理，MySQL提供了函数来处理。

SQL聚集函数

函数	说明
COUNT()	返回某列的行数
MAX()	返回某列最大值
MIN()	返回某列最小值
AVG()	返回某列平均值
SUM()	返回某列值之和

例子：

首先显示出products表格如下：

求出prod_price列的平均值

看起来比较怪，原表只显示了一行：

求出特定行的如vend_id =1003所有商品的价格;

COUNT函数

用于确定满足某种条件的行数目，products表中共有14行：

统计Vend_id = 1001有多少行：

MAX()查找最大值

MIN() 最小值

SUM()求数列值之和

聚集不同的值

统计有多少个厂商，加上了DISTINCT关键字，就只统计该列中不同的值的数量：

组合聚合函数

多个函数可以一起用，功能更强大：

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