SQL Server Exists,not exists usage _mssql

Student table: CREATE TABLE Student
(
ID Number (8) primary key,
Name VARCHAR2, deptment number (8)
)

Selected timetable: CREATE TABLE Select_course
(
ID Number (8) primary key,
STUDENT_ID Number (8) foreign key (course_id) references COURSE (ID),
COURSE_ID Number (8) foreign key (student_id) references STUDENT (ID)
)

Timetable: CREATE TABLE COURSE
(
ID Number (8) is not NULL,
C_name VARCHAR2 (20),
C_no VARCHAR2 (10)
)

 Student table data: 
         ID name             deptment_id 
------------------------------------
           1 echo                    1000 
          2 spring                  
           3 smith                   1000 
          4 liter                   

Data for Course tables:
ID C_name C_no
---------- -------------------- --------
1 Database Data1
2 Mathematical Month1
3 English ENGLISH1

Data for Select_course tables:
ID student_id course_id
---------- ---------- ----------
1 1 1
2 1 2
3 1 3
4 2 1
5 2 2
6 3 2

1. Enquiries took the student ID of all courses, name: (that is, this student does not have a course he did not choose.) ）

and sc.course_id=c.id exist null,

This indicates that (2) The query result in SELECT * from course C does not exist (in 1 queries), and the result of the query returns the course not selected.

The not exists, after the select * from T_student ts, evaluates to FALSE and does not execute the query.

NOT EXISTS
	NOT EXISTS

ID NAME deptment_id
---------- --------------- -----------
1 Echo 1000

2. The query does not select all students who have not selected all courses, that is, students who are not fully elected. (There is a student who has at least one course not selected),

Analysis: As long as there is a door not selected, that is, select * from Select_course SC where student_id=t_student.id and course_id
=course.id has a null, that is, not exists null is true, when the SELECT * from course has the query result (ID is the course.id in the subquery),

Therefore, the select Id,name from t_student executes the query (the ID is t_student.id in the subquery).

Exists

	NOT EXISTS

		(SELECT * from Select_course SC where student_id=t_student.id and course_id=course.id));

ID NAME
---------- ---------------
2 Spring
3 Smith
4 liter

3. A student who is not selected for a course. (without such a student, he has at least one elective course),

Analysis: If he elective a select * From course result set is not NULL, not exists evaluates to false;

The select Id,name from T_student does not execute the query.

NOT EXISTS

	Exists

		(SELECT * from Select_course SC where student_id=t_student.id and course_id=course.id));

ID NAME
---------- ---------------
4 liter

4. Consult a student who has selected at least one course.
Exists

	Exists

		(SELECT * from Select_course SC where student_id=t_student.id and course_id=course.id));

ID NAME
---------- ---------------
1 echo
2 Spring
3 Smith