An interesting SQL Statement Analysis

An interesting SQL Statement Analysis

This statement has plagued me for a long time. It was rarely used in the past due to busy work and work, even if you encounter this requirement, you can use another statement instead or implement it in another solution, so you have no time to ignore it. This Sunday, I had nothing to worry about. I thought carefully and analyzed this statement.

Student table S (SNO int PK, Sn varchar (8) -- SnO indicates the student ID and Sn indicates the Student name.
Curriculum C (CNO int PK, CN varchar (50) -- CNO indicates the course number, and CN indicates the course name
Optional table SC (SNO int PK, CNO int PK, score number (), FK (SNO, CNO) -- score is the score.

-- The following statement is used to find the names of students who have selected all courses.
Select Sn
From S
Where (not exists
(Select *
From C
Where not exists
(Select *
From SC
Where SnO = S. SnO and CNO = C. CNO )))

The analysis is as follows:
[1]:
Select *
From C
Where (not exists
(Select *
From SC
Where SnO = 10005 and CNO = C. CNO ))
-- Find the courses not selected by students whose student ID is 10005.
[2]:
Select Sn from S
Where (not exists ([1] :))
-- If [1]: when some statements are executed as null records, that is, if the not exists ([1] :) condition is true
-- Students whose student ID is 10005 take all the courses, and so on if [1 ]:
-- SnO = 10005 changed to sno = S. Sno, which indicates that the names of the students who took all the courses are obtained.