我的返回结果是返回除一个之外的所有正确答案，没有错误

Question

This is a homework question. I am not asking for the correct answer, I am just looking for help whether I am going in the right direction.

The prompt is that I am to return the first and last name of students who either (OR): (a) Is not currently enrolled (not in Enrollments table) or (b) has the lowest score in any class within their own department.

The database schema is as follows:

My SQL query that I came up with is:

SELECT FIRSTNAME, LASTNAME
FROM STUDENTS
JOIN ENROLLMENTS ON STUDENTS.NETID = ENROLLMENTS.NETID
JOIN COURSES ON COURSES.CRN = ENROLLMENTS.CRN
WHERE STUDENTS.NETID NOT IN (
  SELECT NETID  
  FROM ENROLLMENTS  
) OR
ENROLLMENTS.SCORE IN (
  SELECT MIN(SCORE)  
  FROM ENROLLMENTS  
  WHERE COURSES.DEPARTMENT = STUDENTS.DEPARTMENT  
  GROUP BY ENROLLMENTS.CRN  
);

I tried to use JOIN clauses to combine the three tables where they intersect. The NetId is distinct to each student, identifying them in the Enrollments table. Each class has a unique CRN so I connected them there between Courses table and Enrollments table.

I do not know where the problem lies but the expected outcome is:

Wbixik Yjepuriluwe

Wtoyi Avamijosu

Jheyiresoxo Bsexedoh

Ulerusota Mzuzu

But my outcome is:

Wbixik Yjepuriluwe

Jropop Vduyumi

Jheyiresoxo Bsexedoh

Ulerusota Mzuzu

Looking for any guidance to get me back on track.

Answer 1

You can try using left join

SELECT FIRSTNAME, LASTNAME
FROM STUDENTS
left JOIN ENROLLMENTS ON STUDENTS.NETID = ENROLLMENTS.NETID
JOIN COURSES ON COURSES.CRN = ENROLLMENTS.CRN
where ENROLLMENTS.NETID is null OR
ENROLLMENTS.SCORE IN 
    (SELECT MIN(SCORE) FROM ENROLLMENTS
        WHERE COURSES.DEPARTMENT = STUDENTS.DEPARTMENT
        GROUP BY ENROLLMENTS.CRN
    )