Problems with HAVING query

Question

I am following an online course on databases. However, I don't know how to proceed with this question. Can anyone help? This is my code:

SELECT distinct name
FROM Persons P, Knows K
WHERE K.personA_id = P.id AND K.personB_id = P.id
GROUP BY name
HAVING SUM(K.id) = 2

Answer 1

SELECT distinct p.name
FROM Knows K
LEFT JOIN Persons P,
ON K.personA_id = P.id 
WHERE K.personB_id 
  IN (
    SELECT id
    FROM Persons
    WHERE age>60
  )
GROUP BY name
HAVING COUNT(K.personB_id) = 2