1. List the physicians working in the clinic? (name) 
    SELECT name
FROM physician
  2. List the names and addresses of the patients? (name, address) 
    SELECT name, address
FROM patient
  3. List the dates of all appointments of patient 12345? (date) 
    SELECT DISTINCT date
FROM appointment
WHERE code = '12345'
  4. What are the existing conditions in the database in alphabetical order? (designation) 
    SELECT designation
FROM condition
ORDER BY designation
  5. What patients were seen on January 1, 2007? (number, name) 
    SELECT DISTINCT code, name
FROM appointment JOIN patient USING (code)
WHERE date = '2007-01-01'
  6. What conditions were diagnosed in appointment number 456? (designation) 
    SELECT designation
FROM diagnosed JOIN condition USING (ref)
WHERE num = 456
  7. How many appointments took place on January 1, 2007? (number) 
    SELECT COUNT(*)
FROM appointment
WHERE date = '2007-01-01'
  8. How many times was each room used? (room, number) 
    SELECT room, COUNT(*)
FROM appointment
GROUP BY room
  9. How many times was each room used by the physician with number 99030? (room, number) 
    SELECT room, COUNT(*)
FROM appointment
WHERE number = '99030'
GROUP BY room
  10. How many times was each room used by the physician Luca Moore? (room, number) 
    SELECT room, COUNT(*) as number
FROM appointment join physician using(number)
WHERE name = 'Luca Moore'
GROUP BY room;
  11. What rooms were used more than twice on 1 January 2007? (room) 
    SELECT room
FROM appointment
WHERE date = '2007-01-01'
GROUP BY room
HAVING COUNT(*) > 2
  12. What are the three most used rooms in that same day? (room) 
    SELECT room
FROM appointment
WHERE date = '2007-01-01'
GROUP BY room
HAVING COUNT(*) IN (
  SELECT COUNT(*)
  FROM appointment
  WHERE date = '2007-01-01'
  GROUP BY room
  ORDER BY COUNT(*) DESC
  LIMIT 3
)
  13. What conditions have been diagnosed for patient 12345? (designation) 
    SELECT DISTINCT designation
FROM appointment JOIN diagnosed USING(num) JOIN condition USING (ref)
WHERE code = '12345'
  14. What patients have been diagnosed conditions that have also been diagnosed for patient 12345? (name) 
    SELECT DISTINCT name
FROM appointment JOIN diagnosed USING(num) JOIN patient USING (code)
WHERE ref IN (
  SELECT ref
  FROM appointment JOIN diagnosed USING(num)
  WHERE code = '12345'
)

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