Submitted solutions for Sql3

ConsecutiveNumbers.sql

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+SELECT DISTINCT num AS ConsecutiveNums
+FROM (
+    SELECT num, 
+           LAG(num, 1) OVER (ORDER BY id) AS prev_num,
+           LEAD(num, 1) OVER (ORDER BY id) AS next_num
+    FROM Logs
+) as sub
+WHERE num = prev_num AND num = next_num;

NumberOfPassengers.sql

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+-- Solution.1: 
+WITH CTE AS (
+    SELECT 
+        p.passenger_id, 
+        p.arrival_time, 
+        MIN(b.arrival_time) AS btime 
+    FROM 
+        passengers p 
+    INNER JOIN 
+        buses b 
+    ON 
+        p.arrival_time <= b.arrival_time 
+    GROUP BY 
+        p.passenger_id
+)
+SELECT 
+    b.bus_id, 
+    COUNT(c.btime) AS passengers_cnt 
+FROM 
+    buses b 
+LEFT JOIN 
+    CTE c 
+ON 
+    b.arrival_time = c.btime 
+GROUP BY 
+    b.bus_id 
+ORDER BY 
+    b.bus_id;
+
+-- Solution.2: 
+-- SELECT 
+--     b.bus_id, 
+--     COUNT(p.passenger_id) AS passengers_cnt
+-- FROM 
+--     Buses b
+-- LEFT JOIN 
+--     Passengers p ON p.arrival_time <= b.arrival_time
+-- AND NOT EXISTS (
+--     SELECT 1
+--     FROM Buses b2 
+--     WHERE b2.arrival_time < b.arrival_time AND p.arrival_time <= b2.arrival_time
+-- )
+-- GROUP BY 
+--     b.bus_id
+-- ORDER BY 
+--     b.bus_id;

UserActivity.sql

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+-- Sol.1: 
+
+SELECT activity_date AS 'day', COUNT(DISTINCT user_id) AS 'active_users'
+FROM activity WHERE activity_date > '2019-06-27' AND activity_date <= '2019-07-27'
+GROUP BY 1;
+
+-- Sol.2:
+
+-- SELECT activity_date AS 'day', COUNT(DISTINCT user_id) AS 'active_users'
+-- FROM activity WHERE activity_date BETWEEN '2019-06-28' AND '2019-07-27'
+-- GROUP BY 1;
+
+-- Sol.3:
+
+-- SELECT activity_date AS 'day', COUNT(DISTINCT user_id) AS 'active_users'
+-- FROM activity WHERE DATEDIFF('2019-07-27', activity_date) >= 0 
+-- AND DATEDIFF('2019-07-27', activity_date) < 30
+-- GROUP BY 1;
+
+-- Sol.4: 
+
+-- SELECT activity_date AS 'day', COUNT(DISTINCT user_id) AS 'active_users'
+-- FROM activity WHERE activity_date BETWEEN DATE_SUB('2019-07-27', INTERVAL 29 DAY) 
+-- AND '2019-07-27'
+-- GROUP BY 1;
+
+
+-- Sol.5: 
+
+-- SELECT activity_date AS 'day', COUNT(DISTINCT user_id) AS 'active_users'
+-- FROM activity WHERE activity_date BETWEEN DATE_ADD('2019-07-27', INTERVAL -29 DAY) 
+-- AND '2019-07-27'
+-- GROUP BY 1;