Selecting rows from 3 tables - mysql

Question

I appreciate there are many tutorials and examples out there explaining JOIN concepts- however I am struggling to apply the examples to my specific scenario. I would appreciate some help, and if it isn't too much to ask- a break down of what is going on in the solution to achieve the desired results.
3 Tables: users, assessments, assessment_log.
users
+--------+------+------+-----------+----------+-----------+-------+--------+-----------+---------------+------+----------+
| UserID | User | Pass | FirstName | LastName | LastLogin | Email | Mobile | Kenitalla | AccountStatus | Role | Operator |
+--------+------+------+-----------+----------+-----------+-------+--------+-----------+---------------+------+----------+
assessments
+--------------+------+-----------+-----------+-----------+-----------+-----------+--------------+----------+---------+----------+
| AssessmentID | Name | Criteria1 | Criteria2 | Criteria3 | Criteria4 | Criteria5 | RoleRequired | Required | Renewal | Operator |
+--------------+------+-----------+-----------+-----------+-----------+-----------+--------------+----------+---------+----------+
assessment_logs
+-----------------+------+----------------+------------+--------+-----------+----------+----------+----------+----------+----------+----------+---------+---------------+---------+
| AssessmentLogID | Date | AssessmentName | AssessedBy | UserID | StaffName | Comments | Verdict1 | Verdict2 | Verdict3 | Verdict4 | Verdict5 | Verdict | RenewalPeriod | NextDue |
+-----------------+------+----------------+------------+--------+-----------+----------+----------+----------+----------+----------+----------+---------+---------------+---------+
When a user takes an assessment, an entry goes in to the assessment_log. Some assessments are required (true or false in Required column) where as some are optional. The RoleRequired column stipulates which user Role the assessment is required for.
I would like to generate a list of users and assessments they are yet to pass. The assessments must be required, and the required role must match the users role. An absense of an entry into the assessment_log with a "Pass" in the Verdict column indicates the assessment has not yet been passed.
In plain speak, I am looking for a query that will achieve the following result:
+------------------+-----------------+----------------+
| assessments.Name | users.FirstName | users.LastName |
+------------------+-----------------+----------------+
Where the assessment is required (Required equals true), the RequiredRole matches the users.Role column, and there is no entry in the assessments_log for the assessment where the Verdict column holds a "Pass" value.
Please let me know if further clarification is required.
Thanks in advance!

Answer 1

I'd suggest using a LEFT JOIN.
First, we generate a cross product of assessments and users, which contains for each user, all assessments that are required for their role.
Then, a LEFT JOIN checks if these assessments were passed.
SELECT ...
FROM
users u
JOIN assessments a ON (a.RoleRequired = u.Role)
LEFT JOIN assessment_logs al ON (
al.AssessmentID = a.AssessmentID
AND al.UserID = u.UserID
AND al.Verdict='Pass'
)
WHERE
a.required
AND al.UserID IS NULL

Answer 2

Your attribute names are inconsistent between tables and, although I don't see any duplicate names, it's probably only a matter of time before you do, so I suggest you fix this.
In the meantime, here's how to rename the columns on the fly in order to simplify the join syntax:
SELECT AssessmentName, UserLastName, UserFirstName
FROM ( SELECT FirstName AS UserFirstName, Lastname AS UserLastName, Role
FROM users ) u
NATURAL JOIN
{ SELECT Name AS AssessmentName, RoleRequired AS Role
FROM assessments
WHERE Required ) a
NATURAL JOIN
( SELECT AssessmentName
FROM assessment_logs
WHERE Verdict = 'Pass' ) l;