Consider the following tables:

users                messages
-----------------    -----------------------
user_id  messages    msg_id user_id content
-----------------    -----------------------
1        0           1      1       foo
2        0           2      1       bar
3        0           3      1       foobar
                     4      3       baz
                     5      3       bar

I want to count the number of messages per user and insert the outcome into users.messages, like this:

users                
-----------------    
user_id  messages    
-----------------    
1        3           
2        0           
3        2           

I could use PHP to perform this operation, pseudo:

foreach ($user_id in users) {
  $count = select count(msg_id) from messages where user_id = $user_id
  update users set messages = $count
}

But this is probably very inefficient as compared to one query executed in MySQL directly:

UPDATE users SET messages = (
  SELECT COUNT(msg_id) FROM messages
)

But I'm sure this is not a proper query. Therefore, any help would be appreciated :-)