LIMIT to Percent?

[Nathan Ziarek]

I have three queries, one to grab the most commented stories (LIMIT 10), one to grab the least commented stories (LIMIT 5), and on to grab the most recent stories (LIMIT 10). I am combining these using UNION to then randomly grab half of the resulting set. Is there a way to tell MySQL to "LIMIT 50%" or do I have to run it twice, once for the count and once with a limit?

So...

(SELECT * FROM blog WHERE...) UNION (SELECT * FROM blog WHERE...) UNION (SELECT * FROM blog WHERE...) ORDER BY RAND() LIMIT 50%

Thanks for any resources you can provide!

nate