MySQL order by rand() grouped by day

Question

Is it possible to get random items inside the same day?

For example:

+----+---------------------+  
| id | md                  |  
+----+---------------------+  
|  1 | 2010-06-27 11:26:01 |  
|  2 | 2010-06-27 11:28:20 |  
|  3 | 2010-06-27 11:29:46 |  
|  4 | 2010-06-27 11:30:50 |  
|  5 | 2010-06-27 12:20:56 |  
|  6 | 2010-06-27 12:27:42 |  
|  7 | 2010-06-27 15:14:05 |  
|  8 | 2010-07-06 01:53:33 |  
|  9 | 2010-07-06 01:52:52 |  
+----+---------------------+  

I want to pick random items inside the same day, but at same time i want it ordered by date desc. Something like this:

+----+---------------------+  
| id | md                  |  
+----+---------------------+  

|  8 | 2010-07-06 01:53:33 |  random block  
|  9 | 2010-07-06 01:52:52 |  

|  2 | 2010-06-27 11:28:20 |  random block  
|  4 | 2010-06-27 11:30:50 |  
|  1 | 2010-06-27 11:26:01 |  
|  6 | 2010-06-27 12:27:42 |  
|  3 | 2010-06-27 11:29:46 |  
|  5 | 2010-06-27 12:20:56 |  
|  7 | 2010-06-27 15:14:05 |  

+----+---------------------+  

No idea how to start or if this is even possible since order by rand() won't accept grouping.

Answer 1

You can use a WHERE clause...

WHERE date = 'insert date' ORDER BY rand() LIMIT 1`

Then just use PHP to insert whatever the date is into the string there for each date.

Or to use only one MySQL query, simply order them by date descending and pull them all out using PHP...

while ($item = mysql_fetch_array($query)):
    // process to figure out the date (I'm sure you can do that)
    $mydates[$date][] = $id; // add that ID to that date array
endwhile;
foreach ($mydates as $date):
    $key = array_rand($date);
endforeach;

Answer 2

really simple:

SELECT * 
FROM tbl 
ORDER BY md DESC, RAND()

Answer 3

If I have understood your question correctly, this should do the trick:

SELECT * FROM table1 ORDER BY DATE(md) DESC, RAND();

Example:

DROP TABLE IF EXISTS `table1`;

CREATE TABLE `table1` (
    `id` int(11) NOT NULL auto_increment,
    `md` DATETIME,
    PRIMARY KEY  (`id`)
) ENGINE=InnoDB;

DELIMITER //
DROP PROCEDURE IF EXISTS autofill//
CREATE PROCEDURE autofill()
BEGIN
    DECLARE i INT DEFAULT 0;
    WHILE i < 50 DO
        INSERT INTO table1 (md) VALUES (DATE_ADD(NOW(), INTERVAL FLOOR(RAND() * 1000) HOUR));
        SET i = i + 1;
    END WHILE;
END;
//
DELIMITER ;

CALL autofill();

SELECT * FROM table1 ORDER BY DATE(md) DESC, RAND();

+----+---------------------+
| id | md                  |
+----+---------------------+
|  8 | 2010-08-16 13:46:04 |
| 16 | 2010-08-15 19:46:05 |
| 47 | 2010-08-15 18:46:06 |
| 25 | 2010-08-15 15:46:05 |
| 33 | 2010-08-15 09:46:05 |
|  3 | 2010-08-14 20:46:04 |
| 45 | 2010-08-13 03:46:06 |
| 17 | 2010-08-12 13:46:05 |
| 12 | 2010-08-12 06:46:05 |
|  7 | 2010-08-12 15:46:04 |
| 37 | 2010-08-12 14:46:05 |
| 40 | 2010-08-10 05:46:06 |
| 13 | 2010-08-09 10:46:05 |
|  4 | 2010-08-09 00:46:04 |
|  1 | 2010-08-06 12:46:04 |
| 28 | 2010-08-02 23:46:05 |
| 15 | 2010-08-02 10:46:05 |
|  2 | 2010-08-02 01:46:04 |
| 38 | 2010-07-31 13:46:06 |
| 27 | 2010-07-31 04:46:05 |
| 30 | 2010-07-31 03:46:05 |
| 22 | 2010-07-31 08:46:05 |
| 50 | 2010-07-30 05:46:06 |
| 11 | 2010-07-28 13:46:05 |
| 18 | 2010-07-28 21:46:05 |
| 29 | 2010-07-27 23:46:05 |
| 35 | 2010-07-27 11:46:05 |
|  6 | 2010-07-26 20:46:04 |
| 20 | 2010-07-25 03:46:05 |
| 31 | 2010-07-23 07:46:05 |
| 14 | 2010-07-23 23:46:05 |
| 23 | 2010-07-23 10:46:05 |
| 48 | 2010-07-23 17:46:06 |
| 42 | 2010-07-21 03:46:06 |
| 39 | 2010-07-20 05:46:06 |
| 36 | 2010-07-18 05:46:05 |
| 10 | 2010-07-17 01:46:05 |
| 32 | 2010-07-16 06:46:05 |
|  9 | 2010-07-16 15:46:04 |
| 24 | 2010-07-16 10:46:05 |
| 43 | 2010-07-16 09:46:06 |
|  5 | 2010-07-14 01:46:04 |
| 21 | 2010-07-14 08:46:05 |
| 49 | 2010-07-13 07:46:06 |
| 41 | 2010-07-13 15:46:06 |
| 46 | 2010-07-12 04:46:06 |
| 44 | 2010-07-11 16:46:06 |
| 26 | 2010-07-10 14:46:05 |
| 34 | 2010-07-09 16:46:05 |
| 19 | 2010-07-07 01:46:05 |
+----+---------------------+

Answer 4

Probably not very efficient, but try

select * from (select * from tbl order by rand()) as t group by date(md)