Count number of retweeted tweets and total number tweets have been retweeted
===

Consider the following tables

    users                                tweets
    ---------------------------------    ---------------------------
    user_id num_retweets sum_retweets    tweet_id user_id retweeted
    ---------------------------------    ---------------------------
    1                                    1        1       3
    2                                    2        1       0
    3                                    3        1       4
                                         4        2       0
                                         5        2       0
                                         6        3       1
                                         7        3       2
                                         8        3       0

I want to count `num_retweets`: the number of times a users has written a retweet that has been retweeted and `sum_retweets`: the number of times all of a users tweets have been retweeted. The expected `users` table after the `UPDATE` query is:


    users
    ---------------------------------
    user_id num_retweets sum_retweets
    ---------------------------------
    1       2            7 <-- 3 + 4
    2       0            0
    3       2            3 <-- 1 + 2

Any help on building these two queries would be greatly appreaciated :-) I keep having trouble performing `UPDATE`s across tables.
