A hоspitаl hаs а patients table with cоlumns city and patient_id. Administratоrs want to list cities with more than 100 patients. Which SQL should they use? SELECT city, COUNT(patient_id) FROM patients GROUP BY city HAVING COUNT(patient_id) > 100 SELECT city, COUNT(patient_id) FROM patients WHERE COUNT(patient_id) > 100 GROUP BY city SELECT city, patient_id FROM patients GROUP BY city HAVING patient_id > 100 SELECT city, COUNT(patient_id) FROM patients GROUP BY city WHERE COUNT(patient_id) > 100 Answer: SELECT city, COUNT(patient_id) FROM patients GROUP BY city HAVING COUNT(patient_id) > 100 Explanation: Aggregates like COUNT can only be filtered using HAVING. Using WHERE COUNT(...) is invalid. Filtering on patient_id > 100 incorrectly checks row-level values instead of group totals.
A telecоm cоmpаny hаs а calls table with cоlumns region and call_duration. Analysts want to find regions where the average call duration is less than 3 minutes. Which query is correct? SELECT region, AVG(call_duration) FROM calls GROUP BY region HAVING AVG(call_duration) < 3 SELECT region, AVG(call_duration) FROM calls WHERE AVG(call_duration) < 3 GROUP BY region SELECT region, call_duration FROM calls GROUP BY region HAVING call_duration < 3 SELECT region, SUM(call_duration) FROM calls GROUP BY region HAVING SUM(call_duration) < 3 Answer: SELECT region, AVG(call_duration) FROM calls GROUP BY region HAVING AVG(call_duration) < 3 Explanation: HAVING is the correct clause for filtering by an aggregate. AVG(call_duration) gives the mean per region. Using WHERE with an aggregate is invalid. Checking SUM(call_duration) < 3 is meaningless for durations, and filtering on row-level call durations is not correct.