SQL MIN（）小/大不能正常工作

I have this Data in DB

我在DB中有这个数据

CREATE TABLE Stu_Table
(
    Stu_Id VARCHAR(2),
    Stu_Name VARCHAR(15), 
    Stu_Class VARCHAR(10),
    sub_id VARCHAR(2),
    marks VARCHAR(3)
);

INSERT INTO Stu_Table VALUES (1, 'Komal', 10, 1, 45);
INSERT INTO Stu_Table VALUES (2, 'Ajay', 10, 1, 56);
INSERT INTO Stu_Table VALUES (3, 'Rakesh', 10, 1, 67);
INSERT INTO Stu_Table VALUES (1, 'Komal', 10, 2, 47);
INSERT INTO Stu_Table VALUES (2, 'Ajay', 10, 2, 53);
INSERT INTO Stu_Table VALUES (3, 'Rakesh', 10, 2, 57);
INSERT INTO Stu_Table VALUES (1, 'Komal', 10, 3, 45);
INSERT INTO Stu_Table VALUES (2, 'Ajay', 10, 3, 56);
INSERT INTO Stu_Table VALUES (3, 'Rakesh', 10, 3, 67);
INSERT INTO Stu_Table VALUES (1, 'Komal', 10, 4, 65);
INSERT INTO Stu_Table VALUES (2, 'Ajay', 10, 4, 56);
INSERT INTO Stu_Table VALUES (3, 'Rakesh', 10, 4, 37);
INSERT INTO Stu_Table VALUES (1, 'Komal', 10, 5, 65);
INSERT INTO Stu_Table VALUES (2, 'Ajay', 10, 5, 46);
INSERT INTO Stu_Table VALUES (3, 'Rakesh', 10, 5, 63);
CREATE TA