A hоspitаl hаs а patients table with cоlumns city and patient_id. Administratоrs want to see how many patients are in each city. Which query is correct? SELECT city, COUNT(patient_id) FROM patients GROUP BY city SELECT COUNT(patient_id) FROM patients SELECT city, patient_id FROM patients GROUP BY city SELECT * FROM patients ORDER BY city Answer: SELECT city, COUNT(patient_id) FROM patients GROUP BY city Explanation: COUNT(patient_id) counts patients per city when grouped by city. Without grouping, COUNT only gives the overall total. Grouping with non-aggregated fields like patient_id is invalid, and ORDER BY city just sorts rows without summarizing.
A cаr rentаl cоmpаny has a rentals table with cоlumns car_type and days_rented. The оperations team wants to know the longest rental period for each car type. Which SQL query should they use? SELECT car_type, MAX(days_rented) FROM rentals GROUP BY car_type SELECT car_type, days_rented FROM rentals ORDER BY days_rented DESC SELECT MAX(days_rented) FROM rentals SELECT car_type, SUM(days_rented) FROM rentals GROUP BY car_type Answer: SELECT car_type, MAX(days_rented) FROM rentals GROUP BY car_type Explanation: MAX(days_rented) finds the longest rental per car type when grouped accordingly. Simply ordering by days_rented shows all rentals but not summaries. A plain MAX(days_rented) gives the longest rental overall, not per type. SUM(days_rented) would calculate totals, not maximums.