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create table exam (StudentID int, CourseID int , Grade char(1));
create unique index exam_uix_student_course on exam (StudentId, CourseId);
insert into exam values (1,1,'A'); insert into exam values (2,1,'B'); insert into exam values (3,2,'C');
insert into exam values (1,1,'B');
Duplicate entry '1-1' for key 'exam.exam_uix_student_course'