MySQL每天选择最后一行

Question

Trying to select last row each day.

This is my (simplified, more records in actual table) table:

+-----+-----------------------+------+
| id  |       datetime        | temp |
+-----+-----------------------+------+
|  9  | 2017-06-05 23:55:00   | 9.5  |
|  8  | 2017-06-05 23:50:00   | 9.6  |
|  7  | 2017-06-05 23:45:00   | 9.3  |
|  6  | 2017-06-04 23:55:00   | 9.4  |
|  5  | 2017-06-04 23:50:00   | 9.2  |
|  4  | 2017-06-05 23:45:00   | 9.1  |
|  3  | 2017-06-03 23:55:00   | 9.8  |
|  2  | 2017-06-03 23:50:00   | 9.7  |
|  1  | 2017-06-03 23:45:00   | 9.6  |
+-----+-----------------------+------+

I want to select row with id = 9, id = 6 and id = 3.

I have tried this query:

SELECT MAX(datetime) Stamp
     , temp 
  FROM weatherdata 
 GROUP 
    BY YEAR(DateTime)
     , MONTH(DateTime)
     , DAY(DateTime) 
 order 
    by datetime desc 
 limit 10;

But datetime and temp does not match.

Kind Regards

Answer 1

Here's one way, which gets the MAX date per day and then uses it in the INNER query to get the other fields:

SELECT * 
FROM test
WHERE `datetime` IN (
  SELECT MAX(`datetime`) 
  FROM test
  GROUP BY DATE(`datetime`)
 );

Here's the SQL Fiddle .

Answer 2

If your rows are always inserted and never updated, and if id is an autoincrementing primary key, then

SELECT w.*
  FROM weatherdata w
  JOIN (   SELECT MAX(id) id
             FROM weatherdata
            GROUP BY DATE(datetime)
       ) last ON w.id = last.id

will get you what you want. Why? The inner query returns the largest (meaning most recent) id value for each date in weatherdata . This can be very fast indeed, especially if you put an index on the datetime column.

But it's possible the conditions for this to work don't hold. If your datetime column sometimes gets updated to change the date, it's possible that larger id values don't always imply larger datetime values.

In that case you need something like this.

SELECT w.*
  FROM weatherdata w
  JOIN (   SELECT MAX(datetime) datetime
             FROM weatherdata
            GROUP BY DATE(datetime)
       ) last ON w.datetime = last.datetime

Your query doesn't work because it misuses the nasty nonstandard extension to MySQL GROUP BY. Read this: https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html

It should, properly, use the ANY_VALUE() function to highlight the unpredictability of the results. It shoud read ....

SELECT MAX(datetime) Stamp, ANY_VALUE(temp) temp

which means you aren't guaranteed the right row's temp value. Rather, it can return the temp value from any row in each day's grouping.