1. List the students in the database. (num, name)
    SELECT *
    FROM student
    
  2. In which courses did the student with number 4 enrolled? (ref, year, name)
    SELECT ref, year, name
    FROM course JOIN
      enrollment USING (ref)
    WHERE num = 4
    
  3. In which courses did Michael enroll? (ref, year, name)
    SELECT ref, year, course.name
    FROM student JOIN
      enrollment USING (num) JOIN
      course USING(ref)
    WHERE student.name = 'Michael'
    

    4) How many students are enrolled in the 2007 edition of the SINF course? (total)

    SELECT COUNT(*)
    FROM enrollment JOIN
      course USING(ref)
    WHERE name = 'SINF' AND year = 2007
    

    5) What was the average grade in the first exam of the 2008 edition of the SINF course? (average)

    SELECT ROUND(AVG(grade1), 2)
    FROM enrollment JOIN
      course USING(ref)
    WHERE name = 'SINF' AND year = 2008
    

    6) Which students went to the first SINF exam but did not go to the second one in 2007? (num, name)

    SELECT num, student.name
    FROM student JOIN
      enrollment USING (num) JOIN
      course USING(ref)
    WHERE course.name = 'SINF' AND
       year = 2007 AND
       grade1 IS NOT NULL AND
       grade2 IS NULL
    

    7) Which students scored higher in the second exam than in the first one in the 2007 edition of the SINF course? (num, name)

    SELECT num, student.name
    FROM student JOIN
      enrollment USING (num) JOIN
      course USING(ref)
    WHERE course.name = 'SINF' AND
       year = 2007 AND
       grade1 < grade2
    

    8) For each course, list the average grade of both exams in all editions. (ref, name, average1, average2)

    SELECT course.name, ROUND(AVG(grade1),2), ROUND(AVG(grade2),2)
    FROM enrollment JOIN
      course USING(ref)
    GROUP BY course.ref
    

    9) For each course and year, list the average grade of both exams. (ref, name, year, average1, average2)

    SELECT course.name, year, ROUND(AVG(grade1),2), ROUND(AVG(grade2),2)
    FROM enrollment JOIN
      course USING(ref)
    GROUP BY course.ref, year
    

    10) What was the higher grade, considering both exams, obtained in the SINF course? (grade)

    SELECT MAX(max(grade1, grade2))
    FROM enrollment JOIN
      course USING(ref)
    WHERE name = 'SINF'
    

    11) Who got that grade? (num, name)

Original solution:

SELECT num, student.name
FROM student JOIN
     enrollment USING (num) JOIN
     course USING(ref)
WHERE course.name = 'SINF' AND
      max(grade1, grade2) IN (
        SELECT MAX(max(grade1, grade2))
        FROM enrollment JOIN
             course USING(ref)
        WHERE name = 'SINF'
      )

2019/2020 Solutions:

-- Who got that grade? (num, name)

select student.num, student.name as name, grade1, grade2, course.name
from enrollment
join course on course.ref = enrollment.ref
join student on enrollment.num = student.num
where 
course.name="SINF"
and 
(
	grade1 =
		(
			select max( max(e1.grade1), max(e1.grade2))
			from enrollment e1
			join course c1
			on e1.ref = c1.ref
			where c1.name = "SINF"
		)
	or 
	grade2 =
		(
			select max( max(e2.grade1), max(e2.grade2))
			from enrollment e2
			join course c2
			on e2.ref = c2.ref
			where c2.name = "SINF"
		)
);

or

-- Who got that grade? (num, name)
-- Credits for the query fix: Inês Soares (2019/2020 Student)
-- (Uses Views)

DROP VIEW IF EXISTS max_grade_sinf;
CREATE VIEW max_grade_sinf AS
	SELECT MAX(MAX(grade1), MAX(grade2)) as grade
		FROM
			enrollment
			JOIN course
			ON enrollment.ref = course.ref
			WHERE course.name = 'SINF';

SELECT student.num, student.name
FROM
	enrollment
	JOIN course
	ON enrollment.ref = course.ref
	JOIN student
	ON enrollment.num = student.num
	JOIN max_grade_sinf
	ON (grade1 = grade OR grade2 = grade)
	WHERE course.name = 'SINF';

or (ridiculously long query, just for laughs)

-- Who got that grade? (num, name)
-- "The copy-paste train"

select *
from 
(
	select student.num, student.name as name, grade1, grade2
	from enrollment e1
	join course on course.ref=e1.ref
	join student on e1.num=student.num
	where
	course.name = 'SINF'
	and grade1 is not null and grade1 = ( 
		select max(grade)
		from 
		(
			select grade1 as grade
			from enrollment e2
			join course c2
			on e2.ref=c2.ref
			where c2.name = 'SINF'

			union

			select grade2 as grade
			from enrollment e3
			join course c3
			on e3.ref=c3.ref
			where c3.name = 'SINF'
		)
	)
	union
	select student.num, student.name as name, grade1, grade2
	from enrollment e2
	join course on course.ref=e2.ref
	join student on e2.num=student.num
	where
	course.name = 'SINF'
	and grade2 is not null and grade2 = ( 
		select max(grade)
		from 
		(
			select grade1 as grade
			from enrollment e2
			join course c2
			on e2.ref=c2.ref
			where c2.name = 'SINF'

			union

			select grade2 as grade
			from enrollment e3
			join course c3
			on e3.ref=c3.ref
			where c3.name = 'SINF'
		)
	)
);		

(Credits: André Restivo https://web.fe.up.pt/~arestivo)