[1] Provide another version for the query: 
student information who enrolled in some classes.

[a] output column: student.*
[b] source tables: student: output; enroll: enrollment information
[c] condition:
   a. join condition: enroll.stuId (foreign key in enroll)
   = student.stuId (primary key in student)
   
select distinct s.*
from student s, enroll e  
where s.stuId = e.stuId;


[2] Give a SQL statement for students not enrolled in any class.

Expected: 100003, 100009

[A] All student id.
[a] stuId
[b] student
[c] no
[d] order the result by...

-- [A] All student id
select distinct stuId 
from student
order by stuId;  -- order by clause

select distinct stuId 
from student
order by stuId desc;  -- order by clause

select distinct stuId 
from student
order by stuId desc
limit 5;  

-- [B] All student id of enrolled students.
select distinct e.stuId
from enroll e ;


select distinct s.stuId
from student s, enroll e  
where s.stuId = e.stuId
order by s.stuId;

select distinct s.stuId 
from student s   -- [A]
where s.stuId not in 
-- [B]
(select distinct s.stuId
 from student s, enroll e  
 where s.stuId = e.stuId);
 
select distinct s.*
from student s   -- [A]
where s.stuId not in 
-- [B]
(select distinct s.stuId
 from student s, enroll e  
 where s.stuId = e.stuId);
 
select distinct s.*
from student s   -- [A]
where s.stuId not in 
-- [B] subquery
    (select distinct e.stuId
     from enroll e);