为什么 MYSQL DB 在对 Django models.DateTimeField 进行平均时返回损坏的值?

In MySQL, a DATE or DATETIME expression can be evaluated in a numeric context.

作为一个简单的演示，对 DATETIME 执行 numeric 加法运算会将日期时间值隐式转换为数字.这个查询:

As a simple demonstration, performing an numeric addition operation on a DATETIME implicitly converts the datetime value into a number. This query:

  SELECT NOW(), NOW()+0

返回如下结果:

  NOW()                                NOW()+0  
  -------------------  -----------------------
  2015-06-23 17:57:48    20150623175748.000000

请注意，表达式 NOW()+0 的返回值不是 DATETIME，它是一个数字.

Note that the value returned for the expression NOW()+0 is not a DATETIME, it's a number.

当您在 DATETIME 表达式上指定 SUM() 或 AVG() 函数时，这相当于转换 DATETIME 变成一个数字，然后求和或取平均值.

When you specify a SUM() or AVG() function on a DATETIME expression, that's equivalent to converting the DATETIME into a number, and then summing or averaging the number.

也就是说，这个表达式AVG(mydatetimecol)的返回等价于这个表达式的返回:AVG(mydatetimecol+0)

That is, the return from this expression AVG(mydatetimecol) is equivalent to the return from this expression: AVG(mydatetimecol+0)

平均"是一个数值.而且您已经观察到，返回的值不是有效的日期时间；即使在它看起来像一个有效的日期时间的情况下，它也可能不是一个你会认为是真正的平均值"的值.

What is being "averaged" is a numeric value. And you have observed, the value returned is not a valid datetime; and even in cases where it happens to look like a valid datetime, it's likely not a value you would consider a true "average".

Q2:如果描述的方式失败，我如何获得该字段的实际平均值?

A2:一种方法是将日期时间转换为可以准确"平均的数值，然后将其转换回日期时间.

A2: One way to do that is to convert the datetime into a numeric value that can be "accurately" averaged, and then convert that back into a datetime.

例如，您可以将日期时间转换为表示从某个固定时间点开始的秒数的数值，例如

For example, you could convert the datetime into a numeric value representing a number of seconds from some fixed point in time, e.g.

  TIMESTAMPDIFF(SECOND,'2015-01-01',t.my_date)

然后您可以平均"这些值，以从固定时间点获得平均秒数.(注意:当心添加非常多的行，具有非常大的值，并超过限制(最大数值)，数字溢出问题.)

You could then "average" those values, to get an average number of seconds from a fixed point in time. (NOTE: beware of adding up an extremely large number of rows, with extremely large values, and exceeding the limit (maximum numeric value), numeric overflow issues.)

  AVG(TIMESTAMPDIFF(SECOND,'2015-01-01',t.my_date))

要将其转换回日期时间，将该值作为秒数添加回固定时间点:

To convert that back to a datetime, add that value as a number of seconds back to a the fixed point in time:

  '2015-01-01' + INTERVAL AVG(TIMESTAMPDIFF(SECOND,'2015-01-01',t.my_date)) SECOND

(请注意，DATEIME 值是在 MySQL 会话的时区中评估的；因此在某些极端情况下，MySQL 会话中的 time_zone 变量的设置将对返回的值有一定的影响.)

(Note that the DATEIME values are evaluated in the timezone of the MySQL session; so there are edge cases where the setting of the time_zone variable in the MySQL session will have some influence on the value returned.)

MySQL 还提供了一个 UNIX_TIMESTAMP() 函数，它返回一个 unix 样式的整数值，即从纪元开始(UTC 时间 1970 年 1 月 1 日午夜)开始的秒数.您可以使用它来更简洁地完成相同的操作:

MySQL also provides a UNIX_TIMESTAMP() function which returns a unix-style integer value, number of seconds from the beginning of the era (midnight Jan. 1, 1970 UTC). You can use that to accomplish the same operation more concisely:

  FROM_UNIXTIME(AVG(UNIX_TIMESTAMP(t.my_date)))

请注意，这个最终表达式实际上是在做同样的事情...将日期时间值转换为自 '1970-01-01 00:00:00' UTC 以来的秒数，取其数值平均值，然后将该平均秒数添加回 '1970-01-01' UTC，最后将其转换回 DATETIME 值，以当前会话 time_zone 表示.

Note that this final expression is really doing the same thing... converting the datetime value into a number of seconds since '1970-01-01 00:00:00' UTC, taking a numeric average of that, and then adding that average number of seconds back to '1970-01-01' UTC, and finally converting that back to a DATETIME value, represented in the current session time_zone.

Q3:Django DateTimeField 是否未设置为处理平均?

答: 显然，Django 的作者对从数据库返回的 SQL 表达式 AVG(datetime) 的值感到满意.

A: Apparently, the authors of Django are satisfied with the value returned from the database for a SQL expression AVG(datetime).

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