Why does this query return all the rows?

I have the following code:

SELECT 
    Credit_book_number, 
    (Last_name + ' ' + First_name + ' ' + Patronymic) AS [Name]
FROM 
    Student
WHERE 
    EXISTS (SELECT Credit_book_number
            FROM GradeList g 
            GROUP BY Credit_book_number
            HAVING MIN(g.Grade) >= 6)

I want to get all the students who have a min grade 6, and I need to do it in subquery in where (most wanted to do it with exists, but not obligatory). My query right now returns all the students, however subquery gets not all of them.

Answer

If just one grade satisfies the condition, then this condition is true for all students, because you do not relate it to the current student.

Change it to

SELECT 
    Credit_book_number, 
    (Last_name + ' ' + First_name + ' ' + Patronymic) AS [Name]
FROM 
    Student
WHERE 
    Student.Credit_book_number IN (
            SELECT Credit_book_number
            FROM GradeList g 
            GROUP BY Credit_book_number
            HAVING MIN(g.Grade) >= 6)

Now, only students having a Credit_book_number with an according number in the GradeList satisfying the condition are selected.

Another way to make it work, is to add an additional WHERE clause to the sub-select to connect it to the student.

SELECT 
    Credit_book_number, 
    (Last_name + ' ' + First_name + ' ' + Patronymic) AS [Name]
FROM 
    Student
WHERE 
    EXISTS (SELECT 1   -- It does not matter what we select here
            FROM GradeList g 
            WHERE g.Credit_book_number = Student.Credit_book_number
            GROUP BY Credit_book_number
            HAVING MIN(g.Grade) >= 6)