Question

I have two different tables, my first table contains the authorizations granted to other requests. It has the following columns:

Authorizations table

| authorization_date | role_id | request_id |
|--------------------|---------|------------|
| 2011-08-02         |    1    | 168        |
| 2011-08-10         |    2    | 168        |
| 2011-08-20         |    6    | 168        |
| 2011-08-03         |    2    | 169        |
| 2011-08-24         |    6    | 169        |
| 2011-08-05         |    3    | 170        |
| 2011-08-09         |    5    | 170        |

As you can see, different people have different roles and also can grant a certain level of authorization. The higher the role, the higher the request has been processed.
Now, what I want to do is I want to show the description associated to the role_id (which is in another table) and ONLY the last authorization. Since I have the date for it I already know which one is the latest one. However I don’t know how to do this as I try to group my query by getting the maximum value of the date, when I link it to my second table containing only the description of the role_id, I get duplicates and I just can’t think of a way to do this, I’m kind of new with queries & as I’ve been learning by myself I don’t know many things. Any ideas?

Thanks!

Answer 1

I am guessing a little here, but I think you want:

SELECT Authorizations.*, anotherTable.description
FROM 
(
    SELECT MAX(authorization_date) AS max_date, request_id
    FROM Authorizations
    GROUP BY request_id
) last_auths
INNER JOIN Authorizations ON last_auths.max_date = Authorizations.authorization_date
    AND last_auths.request_id = Authorizations.request_id
INNER JOIN anotherTable ON anotherTable.role_id = Authorizations.role_id

The derived table will get the maximum authorization date for each request. Then you can join to get the role_id for that request and join again to get the description.