138.    Create table Major. Some considerations:

 

q             Grades can be NULL, A, B, C, D, F, I, W

q             A student can not be enrolled in the same class twice

create table enrollment

( classID integer,

  studentID char(9),

  grade char(1),

  constraint enrollment_classstudentID_PK primary key (classID,studentID),

  constraint enrollment_classsID_FK Foreign Key (classID) references class(classID),

  constraint enrollment_studentID_FK Foreign Key (studentID) references student(studentID),

  constraint enrollment_grade_CK check (grade in (NULL,'A','B','C','D','F','I','W')),

  unique (classID, StudentID));