Question

I am trying to merge the data from two tables into one result but without duplicating id’s in the CourseID with the user’s completions taking precedence in the results.

User Course Completion Table

Col Name        | Default | PK?
===================================
UserID                      Y
CourseID                    Y
SystemComplete    NULL      N
UserComplete      NULL      N

UserID, CourseID, SystemComplete, UserComplete
1, 1, NULL, 12/06/2012
1, 2, NULL, 12/06/2012
1, 3, 10/06/2012, 12/06/2012
1, 4, NULL, NULL
1, 5, 10/06/2012, NULL
2, 1, NULL, 12/06/2012

Course to Group table

Col Name        | Default | PK?
===================================
CourseID                     Y
GroupID                      Y

CourseID, GroupID
1, 1
2, 1
3, 1
4, 1
12, 1
5, 2
6, 2

Expected results

I want to find all of the course completions for a user and supplement that list with the uncompleted items. For example, if I want the list for Group 1 and User 1, I would get

CourseID, SystemComplete, UserComplete
======================================
1, NULL, 12/06/2012
2, NULL, 12/06/2012
3, 10/06/2012, 12/06/2012
4, NULL, NULL
12, NULL, NULL --not in completion record

What I’ve tried

I have tried several queries but I can’t seem to figure this out. The one below was something I found in several questions.

SELECT
  COALESCE(uc.CourseID, cg.CourseID) as CourseID
FROM
  CoursesToGroup as cg
FULL JOIN UserCompletion as uc ON
  uc.CourseID = cg.CourseId
WHERE
  uc.UserID = 1 AND
  cg.GroupID = 1

…but this only returns a one row result

The closest I have gotten is UNIONing two queries but then I can’t figure out how to get the UserComplete and SystemComplete data into the query

SELECT CourseID
FROM UserCompletion
WHERE
  EmployeeID = 1

UNION

SELECT CourseID
FROM CoursesToGroup
WHERE
  LearningPathID = 1

This one will return the proper results for the CourseID but, as I stated previously, I can’t figure out how to drop the completion data in there and still remove duplicate CourseId.

What am I missing here?

Answer 1

SELECT CourseID, SystemComplete, UserComplete
FROM UserCompletion
WHERE
UserId = 1

UNION

SELECT CourseId, null, null
FROM CoursesToGroup
where GroupId = 1
and CourseId NOT IN 
    (SELECT CourseId From UserCompletion
     WHERE UserId = 1)