MySQL 按最新时间戳选择

问题描述

我在 SO 上看到了一些类似的问题，但是，我无法找到解决我的具体问题的方法.(仅供参考，这些不是我真正的专栏，只是一个简短的例子).

I've seen some similar types of questions on SO, however, I have not been able to find a solution to my specific issue. (FYI, these are not my real columns, just a shortened example).

我有一个基本表 my_table:

所以，我想做的是能够:

So, what I want to do is be able to:

 1) Select the "newest" row, based on timestamp AND 
 2) Select the 'user_2' column when given a value.  

我尝试过类似的方法:

 SELECT *
 FROM my_table
 WHERE user_2 = 22
 AND timestamp = (
 SELECT MAX( timestamp )
 FROM my_table )
 LIMIT 1 

但这不会返回我正在寻找的行.任何有关修复此查询的帮助都会很棒.

But this does not return the row I am looking for. Any help on fixing this query would be great.

非常感谢.

推荐答案

SELECT * FROM my_table -- standard stuff
   WHERE user_2 = 22 -- predicate
   ORDER BY timestamp DESC -- this means highest number (most recent) first
   LIMIT 1; -- just want the first row

<小时>

顺便说一下，如果您想知道为什么您的原始查询不起作用，让我们分解一下:

By the way, in case you're curious why your original query didn't work, let's break down the pieces:

• 从my_table中选择一些东西...
• 其中 user_2 = 22
• and timestamp = (有些值，暂且搁置)
• 限制 1

• select some stuff from my_table...
• where user_2 = 22
• and timestamp = (some value, let's put it aside for now)
• limit 1

现在，回到 timestamp 值，它来自您的子查询:

Now, coming back to that timestamp value, it comes from your subquery:

SELECT MAX( timestamp ) FROM my_table

请注意，此子查询不限制基于 user_2 的任何行——它询问整个表中的最大时间戳是多少.该最大时间戳是上表中的第一个:(user_1 = 23，user_2 = 25，timestamp = 2012-08-10 22:00:00).

Note that this subquery doesn't restrict any rows based on user_2 -- it asks for what's the max timestamp in the whole table. That max timestamp is the first one in your table above: (user_1 = 23, user_2 = 25, timestamp = 2012-08-10 22:00:00).

那么，让我们把它插回顶级查询:

So, let's plug that back to the top-level query:

• 从my_table中选择一些东西...
• 其中 user_2 = 22
• 和时间戳 = 2012-08-10 22:00:00
• 限制 1

...你可以看到没有这样的一行.

... and you can see there isn't such a row.

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