Dreamfactory : How to use MySQL aggregate functions in queries

SELECT user.id, user.name, COUNT(post.*) AS posts 
FROM user 
  LEFT OUTER JOIN post ON post.owner_id=user.id 
GROUP BY user.id

SELECT P.pid, P.cnt
  FROM Professors P
  WHERE P.cnt <= (SELECT MAX(P2.cnt) FROM Professors P2)

SELECT
  c.name as customer_name,
  sum(coalesce(p.price, 0) * coalesce(pr.quantity, 0)) as amount_purchased
from
  CustomerInfo c
  left join PurchaseRecords pr on c.c_id = pr.c_id
  left join ModelInfo mi on mi.m_id = pr.m_id
  left join ProductInfo p on p.p_id = mi.p_id
group by
  c.name