SQL correct way of joining if the other parameter is null

Question

I have this code and its temporary tables so you can run it.

create table #student
(
    id int identity(1,1),
    firstname varchar(50),
    lastname varchar(50         


        

Answer 1

This

Select Q.id , Q.quiz_name ,S.firstname, S.lastname
from 
    #quiz Q                -- cross join, returns N*K results, do not use without 
    CROSS JOIN #student S  -- where condition that limits it - SAS solution is nicer!
where not exists (select 1 from #quiz_details where quiz_id = Q.id and student_id = S.id)

will give you

id  quiz_name                         firstname     lastname
1   NBA 50 Greatest Player Quiz       LeBron        James
2   NBA Top 10 3 point shooters       Stephen       Curry

---

Edit: changed code to explicit cross join rather then implicit, leaving both in here for comparison

SELECT #quiz Q, # student S           -- old implicit syntax - comma is easily missed

vs.

SELECT #quiz Q CROSS JOIN #student S  -- makes it clearer what is wanted

Answer 2

Pretty sure you want something along these lines. This will give you the quiz values and return NULL for the student and quiz_details when there is no matching data.

select *
from #quiz q
left join #quiz_details qd on q.id = qd.quiz_id
left join #student s on s.id = qd.student_id

Answer 3

My take on it - similar to Patrick's answer with a cross join.

Full sample available at sqlfiddle

select
  Q.Quiz_Name Quiz
 ,S.LastName Last
 ,S.FirstName First
 ,QD.Quiz_ID
 ,QD.Student_ID
from 
/* Get a full list of ALL Test/Student combinations */
           quiz Q 
CROSS JOIN student S 
/* Join the taken tests to the combinations */
 LEFT JOIN quiz_details QD on Q.id = QD.quiz_id
                          and S.id = QD.student_id
/* Only select where no Quiz_ID exists */
WHERE QD.Quiz_ID IS NULL
ORDER BY Q.Quiz_Name, S.Lastname, S.FirstName;

Answer 4

select s.firstname, s.lastname, q.id as not_taken_quiz_id, q.quiz_name as not_taken_quiz_name
from #student s
left join #quiz_details qd on s.id = qd.student_id
left join #quiz q on q.id <> qd.quiz_id

This will give you each student along with the quiz that they have not taken.

Answer 5

This should get you started:

-- filter out the student and quiz you want
DECLARE @qid INT = 1
DECLARE @sid INT = 1

SELECT * 
FROM #student AS s
INNER JOIN #quiz AS q  -- you want the quiz
 ON 1=1
LEFT OUTER JOIN #quiz_details AS qd  -- left join here to get result where rows not found
 ON qd.id = q.id 
 AND qd.student_id=s.id
WHERE s.id = @sid
 AND q.id = @qid
 AND qd.id IS NULL -- only return quizes not taken