SQL query involving group by and joins

Question

I couldn't be more specific in the title part but I want to do something a little bit complex for me. I thought I did it but it turned out that it is buggy.

I have three tables as following:

ProjectTable

• idProject
• title
• idOwner

OfferTable

• idOffer
• idProject
• idAccount

AccountTable

• idAccount
• Username

Now in one query I aim to list all the projects with most offers made, and in the query I also want to get details like the username of the owner, username of the offerer* etc. So I don't have to query again for each project.

Here is my broken query, it's my first experiment with GROUP BY and I probably didn't quite get it.

SELECT Project.addDate,Project.idOwner ,Account.Username,Project.idProject,
    Project.Price,COUNT(Project.idProject) as offercount 
FROM Project 
INNER JOIN Offer 
    ON Project.idProject= Offer.idProject 
INNER JOIN Account 
ON Account.idAccount = Project.idOwner  
GROUP BY Project.addDate,Project.idOwner,
    Account.Username,Project.idProject,Project.Price 
ORDER BY addDate DESC

*:I wrote that without thinking I was just trying to come up with example extra information, that is meaningless thanks to Hosam Aly.

Answer 1

Try this (modified for projects with no offers):

SELECT
  Project.addDate,
  Project.idOwner,
  Account.Username,
  Project.idProject,
  Project.Price,
  ISNULL(q.offercount, 0) AS offercount
FROM
  (
    SELECT
      o.idProject,
      COUNT(o.idProject) as offercount
    FROM Offer o
    GROUP BY o.idProject
  ) AS q
  RIGHT JOIN Project ON Project.idProject = q.idProject
  INNER JOIN Account ON Account.idAccount = Project.idOwner
ORDER BY addDate DESC

Answer 2

I might switch the query slightly to this:

select p.addDate,
  p.idOwner,
  a.Username,
  p.idProject,
  p.price,
  o.OfferCount
from project p
left join
(
  select count(*) OfferCount, idproject
  from offer
  group by idproject
) o
  on p.idproject = o.idproject
left join account a
  on p.idowner = a.idaccount

This way, you are getting the count by the projectid and not based on all of the other fields you are grouping by. I am also using a LEFT JOIN in the event the projectid or other id doesn't exist in the other tables, you will still return data.