Left Join table on condition which may be null and join null columns if condition is null

Question

I have an assignment table which holds assignments created by a teacher in a particular class.

The assignment table has the following fields:

idAssignment (primary key), 
classId (foreign key), 
title, 
assignmentDesc, 
dateDue

I also have an assignmentsubmissions table where students in that class upload their submissions to the assignment.

The assignmentsubmission table has the following fields:

assignmentSubmissionId (primary key), 
assignmentId (foreign key), 
userId (foreign key), 
submission

Basically I want to join assignmentsubmission to assignment for a student in a particular class to see the assignments they have completed and not completed.

I have tried a number of different methods but this is my current SQL command:

SELECT * 
FROM assignment 
  LEFT JOIN assignmentsubmission 
    ON assignment.idAssignment = assignmentsubmission.assignmentId 
WHERE classId = 1234 
    AND ((userId = 1) OR (userId IS NULL))

The student mightn't have yet uploaded a submission to a particular assignment so in that case the userId will not exist for that assignment and I want to join null rows for assignmentsubmission . As it stands all that is happening is only the assignments which have been submitted are showing but I want all assignments, with a submission or null submission fields to be returned.

I'm really struggling with this problem so any help would be greatly appreciated, thanks.

Answer 1

You are on the right track, but the comparison to the student should be in the ON clause:

SELECT a.*, (asu.userId IS NOT NULL) as completed_flag
FROM assignment a LEFT JOIN
     assignmentsubmission asu
     ON a.idAssignment = asu.assignmentId AND
        asu.userId = 1
WHERE a.classId = 1234 ;

This version introduces table aliases which make the query easier to write and to read. Note that all column references are qualified -- meaning the table of origin is identified.