【正确答案】USE Student
   GO
   IF EXISTS(SELECT name FROM sys.indexes
       WHERE name='IX_Teacher_Course_Teacher_id_Course_id_Class_id')
   DROP INDEX IX_Teacher_Course_Teacher_id_Course_id_Class_id ON Teacher_Course
   GO
   CREATE NONCLUSTERED INDEX IX_Teaeher_Course_Teacher id Course_id_Class_id
       ON Teacher_Course(Teacher_id，Course_id,Class_id)