/*（２）连接查询实验

１）　　在学生选课库中实现其数据连接查询操作。

①　　查询每个学生的情况以及他(她)所选修的课程

②　　求学生的学号、姓名、选修的课程及成绩

③　　求选修课程10001且成绩在90分以上的学生学号、姓名及成绩

④　　查询每一门课的间接先行课（即先行课的先行课）

-/* (2) join query in Experiment 1) in the student enrollment database query operation to achieve its data connection. ① check each student' s situation and his (her) students seeking elective courses ② school number, name, elective courses and the results ③ seeking elective courses 10001 and results in more than 90 students in student number, name and achievements ④ check first lesson of each class of indirect (ie first course first course)