Question

Ok, so here is an existing query I have that does what I need.

SELECT C1.id AS id, C1.title AS title, C1.instructor_id AS instructor_id, 
  C1.description AS description, C1.date AS date, C1.starttime AS starttime, 
  C1.endtime AS endtime, C1.training_id AS training_id, C2.name AS name, 
  C2.id AS instructors_id 
FROM trainings C1 
LEFT JOIN instructors C2 ON C1.instructor_id = C2.id

However, I need to add something to it. I have two tables I need to compare in addition to what I have.

Table ‘trainings’ has a list of training sessions, indexed by ‘id’. I also have another table, ‘registrations’, that holds info about what users registered for what training sessions with 3 columns: ‘id’ – index, ‘user_id’ – the ID of the user that registered for a training, and ‘course_id’ – the ID of the training session.

What I need to do in addition to my existing query, is select ONLY the training sessions that do not have a row in ‘registrations’ for that user already.

I hope this makes sense. Thank you for who can help. I have tried for hours. The query is just too large for me to keep organized and to think properly about how to do it.

Answer 1

SELECT C1.id AS id, C1.title AS title, C1.instructor_id AS instructor_id, 
  C1.description AS description, C1.date AS date, C1.starttime AS starttime, 
  C1.endtime AS endtime, C1.training_id AS training_id, C2.name AS name, 
  C2.id AS instructors_id 
FROM trainings C1 
LEFT JOIN instructors C2 ON C1.instructor_id = C2.id
LEFT JOIN registrations C3 ON < Whatever they're connected on >
WHERE C3.id IS NULL

Basicly just JOIN that aswell, and filter it through a WHERE ... IS NULL since LEFT JOIN will return joined table’s columns as NULL.