Mysql return 1 if value found or not at all present, If other values present, return 0

Question

I need to get all the students' details who either are in room number 6 or 7 or not at all in any room. If they are in other rooms, simply, I don't wish to have that record.
My schema is:

students(roll_no, name,class,...)
rooms(room_no,...)
student_room(room_no,roll_no).

students and rooms are related via student_room table. But, I am unable to figure out how to write the query for this.

Answer 1

This will give you details of the students that are in rooms other than 6 or 7:

SELECT
  s.*
FROM
  student AS s
  INNER JOIN student_room AS sr ON s.roll_no = sr.roll_no
WHERE
  sr.room_no NOT IN (6, 7)
;

Now you just need to invert this logic to return the other subset. This is called an anti-join. There is no direct syntax for it in SQL but there are several ways of implementing it using existing syntax. If we take the above query as a starting point, the closest matching anti-join form would be the LEFT JOIN + WHERE IS NULL method:

SELECT
  s.*
FROM
  student AS s
  LEFT JOIN student_room AS sr ON s.roll_no = sr.roll_no
                              AND sr.room_no NOT IN (6, 7)
WHERE
  sr.roll_no IS NULL
;

This is how it works:

1. The result of the join itself will contain all students from the student table, as that table is on the left side of a left outer join.

2. The right side of the join will have data only if the corresponding student is not in room 6 or 7. Otherwise it will contain nulls.

3. The matching rows are excluded in the WHERE clause by checking that there is no match on the right side ( sr.roll_no IS NULL ; it would work the same with sr.roll_no IS NULL provided sr.roll_no cannot have nulls).

Answer 2

I figured it out.

SELECT * FROM students 
INNER JOIN 
(SELECT roll_no,CASE WHEN room_no=6 or room_no=7 THEN 1 
        WHEN room_no IS NULL THEN 1
        ELSE 0 END AS is_room
 from student_room GROUP BY roll_no) AS room_info
ON students.roll_no=room_info.roll_no 
AND room_info.is_room=1