Added SQL solutions

Author: Kyrylo-Ktl

SQL/Actors and Directors Who Cooperated At Least Three Times.sql

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+SELECT actor_id,
+       director_id
+  FROM ActorDirector
+ GROUP BY actor_id, director_id
+HAVING COUNT(*) >= 3;

SQL/Article Views I.sql

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+SELECT DISTINCT author_id AS id
+  FROM Views
+ WHERE author_id = viewer_id
+ ORDER BY id;

SQL/Bank Account Summary II.sql

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+SELECT u.name,
+       SUM(t.amount) AS balance
+  FROM Users AS u LEFT JOIN Transactions AS t
+    ON u.account = t.account
+ GROUP BY u.name
+HAVING SUM(t.amount) >= 10000;

SQL/Calculate Special Bonus.sql

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+SELECT employee_id,
+       IF (employee_id % 2 != 0 AND name NOT LIKE 'M%', salary, 0) AS bonus
+  FROM Employees
+ ORDER BY employee_id;

SQL/Capital GainLoss.sql

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+SELECT stock_name,
+       SUM(IF(operation='Buy', -price, price)) AS capital_gain_loss
+  FROM Stocks
+ GROUP BY stock_name;

SQL/Combine Two Tables.sql

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+SELECT FirstName,
+       LastName,
+       City,
+       State
+  FROM Person AS p LEFT JOIN Address AS a
+    ON p.PersonId = a.PersonId;

SQL/Consecutive Numbers.sql

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+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;

SQL/Customer Placing the Largest Number of Orders.sql

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+SELECT customer_number
+  FROM Orders
+ GROUP BY customer_number
+ ORDER BY COUNT(*) DESC
+ LIMIT 1;

SQL/Customer Who Visited but Did Not Make Any Transactions.sql

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+SELECT v.customer_id,
+       COUNT(*) AS count_no_trans
+  FROM Visits AS v LEFT JOIN Transactions AS t
+    ON v.visit_id = t.visit_id
+ WHERE t.visit_id IS NULL
+ GROUP BY customer_id;

SQL/Customers Who Never Order.sql

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+SELECT name AS 'Customers'
+  FROM Customers
+ WHERE id NOT IN (
+    SELECT customerId
+      FROM Orders
+);

SQL/Daily Leads and Partners.sql

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+SELECT date_id,
+       make_name,
+       COUNT(DISTINCT lead_id) AS unique_leads,
+       COUNT(DISTINCT partner_id) AS unique_partners
+  FROM DailySales
+ GROUP BY date_id, make_name;

SQL/Delete Duplicate Emails.sql

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+DELETE p1
+  FROM Person p1,
+       Person p2
+ WHERE p1.Email = p2.Email
+   AND p1.Id > p2.Id;

SQL/Department Top Three Salaries.sql

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+SELECT d.Name AS Department,
+       a.Name AS Employee,
+       a.Salary
+  FROM (
+    SELECT e.*,
+           DENSE_RANK() OVER (PARTITION BY DepartmentId ORDER BY Salary DESC) AS DepSalRank
+      FROM Employee e
+) a JOIN Department d
+    ON a.DepartmentId = d. Id
+ WHERE DepSalRank <= 3;

SQL/Duplicate Emails.sql

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+SELECT email
+  FROM Person
+ GROUP BY email
+HAVING COUNT(*) > 1;

SQL/Employees Earning More Than Their Managers.sql

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+SELECT emp.name AS Employee
+  FROM Employee emp JOIN Employee man
+    ON emp.managerId = man.id
+ WHERE emp.salary > man.salary;

SQL/Employees With Missing Information.sql

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+WITH Emp AS (
+    SELECT *
+      FROM Employees LEFT JOIN Salaries USING (employee_id)
+     UNION
+    SELECT *
+      FROM Employees RIGHT JOIN Salaries USING (employee_id)
+)
+
+SELECT employee_id
+  FROM Emp
+ WHERE salary IS NULL
+    OR name IS NULL
+ORDER BY employee_id;

SQL/Find Customer Referee.sql

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+SELECT name
+  FROM Customer
+ WHERE referee_id != 2
+    OR referee_id IS NULL;

SQL/Find Followers Count.sql

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+SELECT user_id,
+       COUNT(follower_id) AS followers_count
+  FROM Followers
+ GROUP BY user_id
+ ORDER BY user_id;

SQL/Find Total Time Spent by Each Employee.sql

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+SELECT event_day AS day,
+       emp_id,
+       SUM(out_time - in_time) AS total_time
+  FROM Employees
+ GROUP BY emp_id, event_day;

SQL/Fix Names in a Table.sql

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+SELECT user_id,
+       CONCAT(UPPER(SUBSTRING(name, 1, 1)),
+       LOWER(SUBSTRING(name, 2))) AS name
+  FROM Users
+ ORDER BY user_id;

SQL/Game Play Analysis I.sql

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+SELECT player_id,
+       MIN(event_date) AS first_login
+  FROM Activity
+ GROUP BY player_id
+ ORDER BY player_id;

SQL/Group Sold Products By The Date.sql

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+SELECT sell_date,
+       COUNT(DISTINCT product) AS num_sold,
+       GROUP_CONCAT(DISTINCT product ORDER BY product ASC SEPARATOR ',') products
+  FROM activities
+ GROUP BY sell_date
+ ORDER BY sell_date;

SQL/Market Analysis I.sql

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+SELECT u.user_id AS buyer_id,
+       join_date,
+       COALESCE(COUNT(order_date), 0) AS orders_in_2019
+  FROM Users AS u LEFT JOIN Orders o
+      ON u.user_id = o.buyer_id AND YEAR(order_date) = '2019'
+ GROUP BY u.user_id;

SQL/Not Boring Movies.sql

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+SELECT *
+  FROM Cinema
+ WHERE id % 2 = 1 AND description != "boring"
+ ORDER BY rating DESC;

SQL/Nth Highest Salary.sql

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+CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
+BEGIN
+    SET N = N - 1;
+    RETURN (
+        SELECT DISTINCT(salary)
+          FROM Employee
+         ORDER BY salary DESC
+         LIMIT 1
+        OFFSET N
+  );
+END

SQL/Patients With a Condition.sql

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+SELECT *
+  FROM Patients
+ WHERE conditions REGEXP '\\bDIAB1';

SQL/Rearrange Products Table.sql

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+SELECT product_id,
+       'store1' AS store,
+       store1 AS price
+  FROM Products
+ WHERE store1 IS NOT NULL
+
+ UNION
+
+ SELECT product_id,
+       'store2' AS store,
+       store2 AS price
+  FROM Products
+ WHERE store2 IS NOT NULL
+
+ UNION
+
+ SELECT product_id,
+       'store3' AS store,
+       store3 AS price
+  FROM Products
+ WHERE store3 IS NOT NULL
+
+ ORDER BY product_id, store;

SQL/Recyclable and Low Fat Products.sql

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+SELECT product_id
+  FROM Products
+ WHERE low_fats = 'Y'
+   AND recyclable = 'Y';

SQL/Reformat Department Table.sql

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+SELECT
+    id,
+    MAX(CASE WHEN month = 'Jan' THEN revenue ELSE null END) AS Jan_Revenue,
+    MAX(CASE WHEN month = 'Feb' THEN revenue ELSE null END) AS Feb_Revenue,
+    MAX(CASE WHEN month = 'Mar' THEN revenue ELSE null END) AS Mar_Revenue,
+    MAX(CASE WHEN month = 'Apr' THEN revenue ELSE null END) AS Apr_Revenue,
+    MAX(CASE WHEN month = 'May' THEN revenue ELSE null END) AS May_Revenue,
+    MAX(CASE WHEN month = 'Jun' THEN revenue ELSE null END) AS Jun_Revenue,
+    MAX(CASE WHEN month = 'Jul' THEN revenue ELSE null END) AS Jul_Revenue,
+    MAX(CASE WHEN month = 'Aug' THEN revenue ELSE null END) AS Aug_Revenue,
+    MAX(CASE WHEN month = 'Sep' THEN revenue ELSE null END) AS Sep_Revenue,
+    MAX(CASE WHEN month = 'Oct' THEN revenue ELSE null END) AS Oct_Revenue,
+    MAX(CASE WHEN month = 'Nov' THEN revenue ELSE null END) AS Nov_Revenue,
+    MAX(CASE WHEN month = 'Dec' THEN revenue ELSE null END) AS Dec_Revenue
+  FROM Department
+ GROUP BY id;

SQL/Rising Temperature.sql

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+SELECT DISTINCT a.Id
+  FROM Weather AS a,
+       Weather AS b
+ WHERE a.temperature > b.temperature
+   AND DATEDIFF(a.recordDate, b.recordDate) = 1;

SQL/Sales Analysis III.sql

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+SELECT p.product_id,
+       p.product_name
+  FROM product AS p JOIN sales AS s
+    ON p.product_id = s.product_id
+ GROUP BY s.product_id
+HAVING MIN(s.sale_date) >= '2019-01-01'
+   AND MAX(s.sale_date) <= '2019-03-31';

SQL/Sales Person.sql

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+SELECT s.name
+  FROM SalesPerson s
+WHERE s.sales_id NOT IN (
+    SELECT o.sales_id
+      FROM Orders o LEFT JOIN Company c
+        ON o.com_id = c.com_id
+     WHERE c.name = 'RED'
+);

SQL/Second Highest Salary.sql

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+SELECT (
+    SELECT DISTINCT Salary
+      FROM Employee
+     ORDER BY Salary DESC
+     LIMIT 1
+    OFFSET 1
+) AS SecondHighestSalary;

SQL/Swap Salary.sql

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+UPDATE Salary
+   SET sex = IF (sex = 'm', 'f', 'm');

SQL/The Latest Login in 2020.sql

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+SELECT user_id,
+       MAX(time_stamp) AS last_stamp
+  FROM Logins
+ WHERE YEAR(time_stamp) = 2020
+ GROUP BY user_id;

SQL/Top Travellers.sql

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+SELECT name,
+       COALESCE(SUM(distance), 0) AS travelled_distance
+  FROM Users u LEFT JOIN Rides r
+    ON u.id = r.user_id
+ GROUP BY user_id
+ ORDER BY 2 DESC, 1;

SQL/Tree Node.sql

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+SELECT DISTINCT a.id,
+       CASE
+           WHEN a.p_id IS NULL THEN 'Root'
+           WHEN b.id IS NULL THEN 'Leaf'
+           ELSE 'Inner'
+       END AS Type
+  FROM Tree a LEFT JOIN Tree b
+    ON a.id = b.p_id
+ ORDER BY a.id;

SQL/User Activity for the Past 30 Days I.sql

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+SELECT activity_date as day,
+       COUNT(DISTINCT user_id) as active_users
+  FROM Activity
+ WHERE DATEDIFF('2019-07-27', activity_date) < 30
+   AND activity_date <= '2019-07-27'
+ GROUP BY activity_date;