groupby 为什么要配合聚合函数
首先系统根据SELECT 语句得到一个结果集，如最开始的那个水果、出产国家、单价的一个详细表。然后根据分组字段，将具有相同分组字段的记录归并成了一条记录。这个时候剩下的那些不存在于Group By语句后面作为分组依据的字段就有可能出现多个值，但是目前一种分组情况只有一条记录，一个数据格是无法放入多个数值的，所以这里就需要通过一定的处理将这些多值的列转化成单值，然后将其放在对应的数据格中，那么完成这个步骤的就是聚合函数。这就是为什么这些函数叫聚合函数(aggregate functions)了。
这句话中 后面为啥用聚合函数怎么理解
回复内容：
首先系统根据SELECT 语句得到一个结果集，如最开始的那个水果、出产国家、单价的一个详细表。然后根据分组字段，将具有相同分组字段的记录归并成了一条记录。这个时候剩下的那些不存在于Group By语句后面作为分组依据的字段就有可能出现多个值，但是目前一种分组情况只有一条记录，一个数据格是无法放入多个数值的，所以这里就需要通过一定的处理将这些多值的列转化成单值，然后将其放在对应的数据格中，那么完成这个步骤的就是聚合函数。这就是为什么这些函数叫聚合函数(aggregate functions)了。
这句话中 后面为啥用聚合函数怎么理解
如果没有聚合函数，group的行在不被group的列中的取值将会是第一条or最后一条（忘了），如果有聚合函数，那些列可以根据你的需求进行取值
group by如果不配合聚合函数的话，那么group by 就没什么作用了。
“聚合”顾名思义，将多个值聚在一起进行一些运算，group by 就是讲那些值聚集起来
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