Return a random row without using ID, MySQL

Question

I am looking for a better way of doing this:

SELECT * FROM $tbl_name WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM $tbl_name ) ORDER BY id LIMIT 1;

It is slow and isn't very random, I get the same result every 10 or so query's.

This selects a random row from the table regardless of its ID.

So far I have this:

// Get amount of rows in database
$result = mysql_query("SELECT * FROM $tbl_name");
$num_rows = mysql_num_rows($result);

// Generate random number
$random_row = rand(1, $num_rows);

But I don't know how to get a certain row.

I do not mean

SELECT * FROM $tbl_name WHERE id = $random_row

Because my database has gaps in the ID column so it would sometimes fail.

Has anyone got script that can get a random result from a MySQL database without replying on IDs and is super fast? (the database contains about 20000 rows)

Answer 1

SELECT * FROM $tbl_name WHERE 1 ORDER BY RAND() LIMIT 1;

20,000行并不是很多，以上应该足够快。

Answer 2

Juhana is right by the book: "ORDER BY RAND() LIMIT 1", and of course 20k isn1t that much. Other option will be with subselects: SELECT [fields] FROM myTable, (SELECT FLOOR(MAX(myTable.id) * RAND()) AS randId FROM myTable) AS someRandId WHERE myTable.id = someRandId.randId

discussed here (please avoid select * when it`s unnecessary)

Answer 3

After some searching though the comments on the link Adi sent, I have found a decent solution.

SELECT * FROM $tbl_name T JOIN (SELECT CEIL(MAX(ID)*RAND()) AS ID FROM $tbl_name) AS x ON T.ID >= x.ID LIMIT 1; 

Source: wanderr.com/jay/order-by-slow/2008/01/30

Seems to be very fast and very random!