SQL self-connection-Example: querying the first course of the first course

There is a course table with course No. (CNO), course name (cname), and first course No. (pcno)

Now I want to query the first course of each course. Solution (self-connection ):

Consider the Course table as A and B. In this way, it is equivalent to connecting two tables. The control condition is that the course number in Table A is equal to the first course number in table B. The course number in Table A is the "grandpa" of the course number in Table B, that is, the first course.

SQLCode: Select a. CNO, A. cname, B. pcno
From course A, course B
Where a. pcno = B. CNO