Select StudentGrade,Sum(Student.StudentCode) AS Number
From Student,teach
Where teach.StudentCode=Student.StudentCode AND Mark>=17
Group By StudentGrade

Select StudentGrade,Sum(Student.StudentCode) AS Number
From Student,teach
Where teach.StudentCode=Student.StudentCode AND Mark>=17

Select StudentGrade,Count(Student.StudentCode) AS Number
From teach,Student
Where teach.StudentCode=Student.StudentCode AND Mark>=17

Select StudentGrade,Count(Student.StudentCode) AS Number
From teach,Student
Where teach.StudentCode=Student.StudentCode AND Mark>=17
Group By StudentGrade

            int i = 1;
            string s;
            for (s = "A"; i <= 3; s += "B")
                s += ++i;
            Console.WriteLine(s);
            Console.ReadKey();

AB2B3B4

A1B2B3B

A2B3B4B

A2                       
A2B3                   
A2B3B4               
A2B3B4B