/*
Microsoft SQL Interview Question

Considering a dataset that tracks user interactions with different clients, identify which clients have users who are exclusively loyal to them (i.e., they don't interact with any other clients).

For each of these clients, calculate the number of such exclusive users. The output should include the client_id and the corresponding count of exclusive users.
*/
    
-- Schema Setup
CREATE TABLE meetup_events (client_id VARCHAR(255), customer_id VARCHAR(255), event_id BIGINT, event_type VARCHAR(255), id BIGINT PRIMARY KEY, time_id DATETIME, user_id VARCHAR(255));

INSERT INTO meetup_events (client_id, customer_id, event_id, event_type, id, time_id, user_id) VALUES ('C001', 'CU001', 101, 'click', 1, '2025-01-01 10:00:00', 'U001'), ('C001', 'CU002', 102, 'view', 2, '2025-01-01 11:00:00', 'U002'), ('C002', 'CU003', 103, 'click', 3, '2025-01-02 10:00:00', 'U003'), ('C002', 'CU003', 104, 'view', 4, '2025-01-02 11:00:00', 'U003'), ('C003', 'CU004', 105, 'click', 5, '2025-01-03 10:00:00', 'U004'), ('C001', 'CU001', 106, 'view', 6, '2025-01-04 10:00:00', 'U001'), ('C003', 'CU005', 107, 'click', 7, '2025-01-05 10:00:00', 'U005'), ('C004', 'CU006', 108, 'view', 8, '2025-01-06 10:00:00', 'U006'), ('C004', 'CU006', 109, 'click', 9, '2025-01-07 10:00:00', 'U006'), ('C001', 'CU007', 110, 'click', 10, '2025-01-08 10:00:00', 'U007');

-- Solution
SELECT
    client_id,
    COUNT(DISTINCT user_id) AS exclusive_users
FROM meetup_events
WHERE user_id IN (SELECT user_id FROM meetup_events GROUP BY user_id HAVING COUNT(DISTINCT client_id) = 1)
GROUP BY client_id;