GATE | Sudo GATE 2020 Mock III (24 January 2019) | Question 61

Consider the following schema:

Person(SSN, name, address)
Car(license, year, model)
Accident(license, accident_date, driver, damage_amount)
Owns(SSN, license) 

Primary keys are underlined. Note that the driver involved in a car accident may not always be the owner of the car. Assume that accident_date is of type integer, and represents a year (e.g. 1980). Year is also of type integer. Also, assume that a car cannot get involved in more than one accident at a certain date. Which of the following query is correct to find the SSN of every person who owns one or more cars, none of which has ever been involved in a car accident.

SELECT O.SSN
FROM Owns O
WHERE O.license NOT IN (SELECT A.license
                        FROM Accident A) 

πSSN(Owns) - πSSN(Owns ⨝ Accident) 

{ O | ∃O1 ∈ Owns( O.SSN = O1.SSN ∧ ¬∃A∈ Accident( O1.license = A.license )) }