Comments on: “Order By” in SQL queries and random selection of database items http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/ The Science of Web Art, Design and Development Tue, 06 Jan 2009 23:45:55 +0000 http://wordpress.org/?v=2.7 hourly 1 By: guioconnor http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-1030 guioconnor Fri, 18 Jan 2008 08:49:31 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-1030 <p>@Benjamin Wilger - <blockquote>Nice article and thanks for the credit! (thank you, google!) But it's not worth to give the credit to me since all that is written in the MySQL Manual ;-)</blockquote></p> <p>Hi, Ben. I wanted to drop you a line back when I wrote this post, but I couldn't find you around. Now, I think credit is due, it was a nice solution when showed it up :-)</p> <blockquote>A little hint for the ORDER BY rand() stuff (...) if you have a fairly big table (e.g. more than 1000 entries) you might get some performance problems.</blockquote> <p>Thanks for the additional hint!</p> @Benjamin Wilger -
Nice article and thanks for the credit! (thank you, google!) But it’s not worth to give the credit to me since all that is written in the MySQL Manual ;-)

Hi, Ben. I wanted to drop you a line back when I wrote this post, but I couldn’t find you around. Now, I think credit is due, it was a nice solution when showed it up :-)

A little hint for the ORDER BY rand() stuff (…) if you have a fairly big table (e.g. more than 1000 entries) you might get some performance problems.

Thanks for the additional hint!

]]> By: Benjamin Wilger http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-1023 Benjamin Wilger Sun, 13 Jan 2008 13:23:20 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-1023 <p>Guten Tag!</p> <p>Nice article and thanks for the credit! (thank you, google!) But it's not worth to give the credit to me since all that is written in the MySQL Manual ;-)</p> <p>A little hint for the ORDER BY rand() stuff: Keep in mind that MySQL optimizes LIMITed Queries. So if you say LIMIT 2 it will just <em>read</em> two entries from the table unless you do an ORDER BY. If you order by an indexed column the job is simple for MySQL. But if you sort by a value produced by a function like rand(), MySQL is forced to do a full table scan to get all the rows. It then needs to copy the unsorted result to memory (if too big it writes them to hard disc) and sorts them. So if you have a fairly big table (e.g. more than 1000 entries) you might get some performance problems.</p> <p>Greetings, Benjamin</p> Guten Tag!

Nice article and thanks for the credit! (thank you, google!) But it’s not worth to give the credit to me since all that is written in the MySQL Manual ;-)

A little hint for the ORDER BY rand() stuff: Keep in mind that MySQL optimizes LIMITed Queries. So if you say LIMIT 2 it will just read two entries from the table unless you do an ORDER BY. If you order by an indexed column the job is simple for MySQL. But if you sort by a value produced by a function like rand(), MySQL is forced to do a full table scan to get all the rows. It then needs to copy the unsorted result to memory (if too big it writes them to hard disc) and sorts them. So if you have a fairly big table (e.g. more than 1000 entries) you might get some performance problems.

Greetings, Benjamin

]]> By: guioconnor http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-473 guioconnor Tue, 07 Aug 2007 14:54:34 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-473 <p>@Andy - :-) I'm glad it has been useful</p> @Andy - :-) I’m glad it has been useful

]]> By: Andy http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-470 Andy Tue, 07 Aug 2007 10:38:43 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-470 <p>Spot on with that last tip - thanks! The MySQl manual is not great and I was despairing of Google until now.</p> Spot on with that last tip - thanks! The MySQl manual is not great and I was despairing of Google until now.

]]> By: guioconnor http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-390 guioconnor Fri, 15 Jun 2007 10:44:46 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-390 <p>@Oda: That is indeed a useful link, I updated the article to include it. Thanks!</p> <p>@AndreYK: Great observation, thanks for sharing it.</p> @Oda: That is indeed a useful link, I updated the article to include it. Thanks!

@AndreYK: Great observation, thanks for sharing it.

]]> By: AndreYK http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-389 AndreYK Thu, 14 Jun 2007 23:53:38 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-389 <p>I have a tip too ! It is very common to use GROUP BY queries. However, when we use it MySQL sorts the entries by default, spending some computational time. If you wish to optimize GROUP BY queries, it is interesting to not sort using ORDER BY NULL. See the link that Oda cited.</p> I have a tip too ! It is very common to use GROUP BY queries. However, when we use it MySQL sorts the entries by default, spending some computational time. If you wish to optimize GROUP BY queries, it is interesting to not sort using ORDER BY NULL. See the link that Oda cited.

]]> By: oda http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-388 oda Thu, 14 Jun 2007 21:00:20 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-388 <p>Hei! Very good article! The last example was indeed remarkable!</p> <p>MySQL is full of nice functions, take a look at: http://dev.mysql.com/doc/refman/5.0/en/functions.html</p> <p>And don't forget: RTFM</p> Hei! Very good article! The last example was indeed remarkable!

MySQL is full of nice functions, take a look at: http://dev.mysql.com/doc/refman/5.0/en/functions.html

And don’t forget: RTFM

]]> By: guioconnor http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-387 guioconnor Thu, 14 Jun 2007 18:42:00 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-387 <p>Thanks Andre. I didn't know that either until some weeks ago :-)</p> <p>When I first saw it, it really expanded my mind.</p> Thanks Andre. I didn’t know that either until some weeks ago :-)

When I first saw it, it really expanded my mind.

]]> By: Andre http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/comment-page-1/#comment-386 Andre Thu, 14 Jun 2007 18:38:22 +0000 http://www.z-oc.com/blog/2007/06/order-by-in-sql-queries-and-random-selection-of-database-items/#comment-386 <p>Very nice tips about "ORDER by". The last example was new to me, i have never thought about boolean formulas with "ORDER by" queries.</p> Very nice tips about “ORDER by”. The last example was new to me, i have never thought about boolean formulas with “ORDER by” queries.

]]>