Select the difference between the maximum and minimum weight of a person with the last name Maroni.

Question

SELECT weight AS weight_delta
from (select max(weight) 
      from patients 
      where last_name='Maroni')
      -
      (select min(weight) 
      from patients 
      where last_name='Maroni');

Answer 1

You are thinking too complicatedly. Find the difference without any subquery:

SELECT MAX(weight) - MIN(weight) AS weight_delta 
FROM patients WHERE last_name = 'Maroni';