Update statement with select - help please ;)

[Gavin Reynolds]

Hi, really easy one here i'm sure but i cannot seem to get it to work

table asd_a (userid | name | average)
table asd_b (userid | rating)

asd_a contains a unique userid (e.g.001)
asd_b stores multiple ratings (1 to 5) against the userid

I can display the average score for each userid;
select round((sum(rating)/count(rating)),1) as average from asd_b where id = '001';

what I want to do is UPDATE the 'average' field in asd_a for each of the userid's

Any help would be greatly appreciated