Sqlserver exists, not exists usage

Student table: create table student
(
Id number (8) primary key,
Name varchar2 (10), deptment number (8)
)

Course Selection table: 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)
)

COURSE schedule: create table COURSE
(
Id number (8) not null,
C_NAME VARCHAR2 (20 ),
C_NO VARCHAR2 (10)
)

Student table data:
Id name DEPTMENT_ID
------------------------------------
1 echo 1000
2 spring 2000
3. smith 1000
4 liter 2000

Course table data:
ID C_NAME C_NO
--------------------------------------
1 Database data1
2. month1
3 English english1

Data in the select_course table:
ID STUDENT_ID COURSE_ID
------------------------------
1 1 1
2 1 2
3 1 3
4 2 1
5 2 2
6 3 2

1. query the student id and name that have taken all courses: (that is, this student does not have a course that he has not selected .)

Analysis: If no course is selected, select * from select_course SC where SC. student_id = ts. id

And SC. course_id = c. id null,

This indicates that (2) the query result of select * from course c does not exist (1 query), and the query result returns the course that is not selected,

In this case, the not exists after select * from t_student ts determines that the result is false and no query is executed.

SQL> select * from t_student ts where not exists
 (select * from course c where not exists
  (select * from select_course sc where sc.student_id=ts.id and sc.course_id=c.id));        

        ID NAME            DEPTMENT_ID
---------- --------------- -----------
         1 echo                   1000

2. query the students who have not selected all courses. (If such a student exists, he does not select at least one course ),

Analysis: select * from select_course SC where student_id = t_student.id and course_id
= Either course. id is null, that is, not exists null is true. In this case, select * from course has the query result (id is the course. id in the subquery ),

Therefore, select id, name from t_student will execute the query (id is t_student.id in the subquery ).

SQL> select id,name from t_student where exists

(select * from course where 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. query the students who have not selected a course. (If such a student is not saved, he must take at least one course ),

Analysis: if he chooses a select * from course result set that is not empty, not exists determines that the result is false;

Select id, name from t_student does not execute the query.

SQL> select id,name from t_student where not exists

(select * from course where exists

(select * from select_course sc where student_id=t_student.id and course_id=course.id));

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

4. query the students who take at least one course.
SQL> select id, name from t_student whereExists

(select * from course where  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