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So, for all of us who are used to "ORDER BY RAND() LIMIT 1" -- which doesn't scale well at all -- here's a "better" way.


With a JOIN (concept from Jan Kneschke): 

tablename AS t1
    ROUND(RAND() * (SELECT MAX(id) FROM tablename)) AS id
) AS t2 
t1.id >= t2.id 
t1.id ASC 


Or with subqueries:

SELECT * FROM tablename
WHERE id >= FLOOR( RAND( ) * ( SELECT MAX( id ) FROM tablename ) )


In principle, this can be extended to N rows... just remember that it's possible than <N rows are retrieved, depending on the distribution of id values -- so if it really matters that you get N back, you may want to ask for some more, just in case.  Either way, you'll still have to check the size of the result set.


Hope that helps.

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This page contains some in-depth, extremely well-written articles on query optimization in very common situations.


A collection of fantastic resources on the subject of MySQL Performance:

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MySQL's tech resource article on database normal forms is an excellent read.


NEW!!! - A large (160MB, 4M record) sample database with test suite for MySQL -- fantastic for running "real" queries.


In addition, MySQL provides a number of sample databases for testing purposes; personally, I find this one to be the most useful for playing around with SQL statements and such.


The Zip Code Database Project exists to provide US Zip Codes in their entirety; latitude and longitude coordinates included! The downloads are in CSV and MySQL table dump formats.


Also, there's an pretty good online "data generator", and SQL is one of the options... although personally, I prefer the integers table approach (courtesy of Baron Schwartz):


set @num_gamers    := 10000,
    @num_countries := 5,
    @num_games     := 10;

drop table if exists gamer;
drop table if exists game;
drop table if exists country;
drop table if exists score;
drop table if exists semaphore;

create table gamer(
   gamer int not null,
   country int not null,
   name varchar(20) not null,
   primary key(gamer)

create table game(
   game int not null,
   name varchar(20) not null,
   primary key(game)

create table score(
   gamer int not null,
   game int not null, 
   score int not null, 
   primary key(gamer, game),
   index(game, score),

create table country(
   country int not null,
   name varchar(20) not null,
   primary key(country)

-- I use the integers table to generate large result sets.
drop table if exists integers;
create table integers(i int not null primary key);
insert into integers(i) values(0),(1),(2),(3),(4),(5),(6),(7),(,(9);

insert into country(country, name)
   select t.i * 10 + u.i, concat('country', t.i * 10 + u.i)
   from integers as u
      cross join integers as t
   where t.i * 10 + u.i < @num_countries;

insert into game(game, name)
   select t.i * 10 + u.i, concat('game', t.i * 10 + u.i)
   from integers as u
      cross join integers as t
   where t.i * 10 + u.i < @num_games;

insert into gamer(gamer, name, country)
   select th.i * 1000 + h.i * 100 + t.i * 10 + u.i,
      concat('gamer', th.i * 1000 + h.i * 100 + t.i * 10 + u.i),
      floor(rand() * @num_countries)
   from integers as u
      cross join integers as t
      cross join integers as h
      cross join integers as th;

insert into score(gamer, game, score)
   select gamer.gamer, game.game, floor(rand() * @num_gamers * 10)
   from gamer
      cross join game;

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Hopefully this presentation will stay online at scribd...  it's simply fantastic, probably the best I've come across in recent memory.  At 220 slides, it's quite lengthy -- but the lessons learned are invaluable, so be sure to read all the way to the end.




EDIT: This year's version of the presentation -- some really great stuff in here, particuarly about hierarchies.

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A four part tutorial that explains the PHP / MySQL extensions - mysql, mysqli, and pdo_mysql - with simple examples is now accessible from Sun Developer Network.


Application developers who are new to the development of MySQL database applications with PHP are the target audience of this tutorial.



  • Part 1: Using the MySQL Improved Extension, mysqli
  • Part 2: Using the MySQL Extension, mysql
  • Part 3: Using the PDO Extension With MySQL Driver, pdo_mysql
  • Part 4: Using the MySQL Native Driver for PHP, mysqlnd


Tutorial is here.

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