completed sql3

problem_1.sql

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+--consecutive numbers 
+
+SELECT DISTINCT l1.num  AS 'ConsecutiveNums' FROM logs l1, logs l2, logs l3
+WHERE l1.id= l2.id - 1
+AND l2.id = l3.id - 1
+AND l1.num = l2.num
+AND l2.num = l3.num;

problem_2.sql

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+-- Number of Passengers in Each Bus
+
+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;

problem_3.sql

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+--User Activity
+
+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 activity_date;

problem_4.sql

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+--Dynamic Pivoting of a Table
+
+CREATE PROCEDURE PivotProducts()
+BEGIN
+    -- Set the maximum length for GROUP_CONCAT
+    SET SESSION GROUP_CONCAT_MAX_LEN = 1000000;
+
+    -- Generate dynamic SQL for pivoting
+    SELECT GROUP_CONCAT(
+        DISTINCT CONCAT('SUM(IF(Store = "', store, '", Price, NULL)) AS ', store)
+    ) INTO @SQL
+    FROM Products;
+
+    -- Construct the final SQL query
+    SET @SQL = CONCAT(
+        'SELECT product_id, ', @SQL, ' FROM Products GROUP BY product_id'
+    );
+
+    -- Prepare and execute the SQL statement
+    PREPARE stmt FROM @SQL;
+    EXECUTE stmt;
+    DEALLOCATE PREPARE stmt;
+END