update a random row from a table

Question

I've got this code here:

    UPDATE starinformation SET starOwner = -1 WHERE (
SELECT starinformation.starID
ORDER BY RAND( ) 
LIMIT 1
)

I want to update a random row from a table with information pulled from that table and only effect 1 row.

Answer 1

There are a lot of problems with this approach:

• MySQL does not allow combining a modifying outer query with a selecting subquery to touch the same table
• ORDER BY RAND() LIMIT 1 is a major performance killer: It will create an intermediate result of ALL rows - ie if your table contains 1 megarows, you will touch every single one of them.

My recommendation is to

• allways have a sortable single-field PK
• Run SELECT ROUND(RAND()*COUNT(*)) AS num FROM starinformation , returning $num
• Run UPDATE starinformation SET starOwner = -1 ORDER BY [pk-field] LIMIT $num,1