CREATE TABLE tweets (id NUMBER, uid NUMBER, text CLOB, createdate TIME)
CREATE TABLE user_profiles (uid NUMBER, name CLOB, email CLOB, partitionid NUMBER, followers NUMBER)
CREATE TABLE follows (f1 NUMBER, f2 NUMBER)

Find the average number of followers for the users who had some tweets.
SELECT AVG(followers) FROM user_profiles WHERE uid IN (SELECT uid FROM tweets)
SELECT AVG("followers") FROM "user_profiles" WHERE "uid" IN (SELECT "uid" FROM "tweets")