sql - SQL：具有 DATE 类型列的聚合函数

Question

我偶然发现了一个关于哪些聚合函数适用于 DATE 类型列的问题（在测试中）。因此，据我了解， COUNT 只会计算行数，而 MIN 和 MAX 返回最早/最晚日期。但是，我对 SUM 和 AVG 函数有点困惑。他们是否会将 DATE 值转换为整数并计算这些整数的总和/平均值？还是我在这里错了？无论如何，这种行为在所有 SQL 实现中是否一致？提前致谢。

Answer 1

In MS SQL Server You cannot call the SUM operator on datetime types, nor can you call the AVG operator.

MSDN lists the return types of the SUM operator here:
http://msdn.microsoft.com/en-us/library/ms187810.aspx

It is also a valid reference for the types on which you may invoke the SUM operator.

EDIT: In response to your comment, you may use a site like sqlfiddle to test various implementations

http://www.sqlfiddle.com/#!3/22cee/1

Considering the backend storage of datetime is not standardized, I contend that it should not be depended upon for any database to return SUM or AVG results in predictable ways.... better not to do it at all...