Solutions to super hard LeetCode Database questions (all free ones)

Author: Dominik Schauer

database/1179_Reformat_Department_Table.txt

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+/*Pivot solution
+
+SELECT 
+    id, 
+    Jan AS Jan_Revenue,
+    Feb AS Feb_Revenue,
+    Mar AS Mar_Revenue,
+    Apr AS Apr_Revenue,
+    May AS May_Revenue,
+    Jun AS Jun_Revenue,
+    Jul AS Jul_Revenue,
+    Aug AS Aug_Revenue,
+    Sep AS Sep_Revenue,
+    Oct AS Oct_Revenue,
+    Nov AS Nov_Revenue,
+    Dec AS Dec_Revenue
+FROM department
+PIVOT 
+(
+    SUM(revenue)
+    FOR month
+    IN ([Jan],[Feb],[Mar],[Apr],[May],[Jun],[Jul],[Aug],[Sep],[Oct],[Nov],[Dec])
+)
+as pivot*/
+
+SELECT
+	id,
+	SUM(CASE WHEN month = 'Jan' THEN revenue END) AS Jan_Revenue,
+	SUM(CASE WHEN month = 'Feb' THEN revenue END) AS Feb_Revenue,
+	SUM(CASE WHEN month = 'Mar' THEN revenue END) AS Mar_Revenue,
+	SUM(CASE WHEN month = 'Apr' THEN revenue END) AS Apr_Revenue,
+	SUM(CASE WHEN month = 'May' THEN revenue END) AS May_Revenue,
+	SUM(CASE WHEN month = 'Jun' THEN revenue END) AS Jun_Revenue,
+	SUM(CASE WHEN month = 'Jul' THEN revenue END) AS Jul_Revenue,
+	SUM(CASE WHEN month = 'Aug' THEN revenue END) AS Aug_Revenue,
+	SUM(CASE WHEN month = 'Sep' THEN revenue END) AS Sep_Revenue,
+	SUM(CASE WHEN month = 'Oct' THEN revenue END) AS Oct_Revenue,
+	SUM(CASE WHEN month = 'Nov' THEN revenue END) AS Nov_Revenue,
+	SUM(CASE WHEN month = 'Dec' THEN revenue END) AS Dec_Revenue
+FROM department
+GROUP BY id

database/175_Combine_Two_Tables.txt

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+SELECT FirstName, LastName, City, State
+FROM person AS p
+LEFT JOIN address AS a 
+ON p.personId = a.personId

database/176_Second_Highest_Salary.txt

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+WITH
+ranks AS
+(SELECT
+    Salary, 
+    RANK() OVER (ORDER BY Salary DESC) as rank
+FROM Employee),
+result AS
+(SELECT Salary AS SecondHighestSalary
+FROM ranks
+WHERE rank = 2)
+SELECT ISNULL(
+        (SELECT TOP 1 SecondHighestSalary FROM result),
+        NULL) AS SecondHighestSalary

database/177_Nth_Highest_Salary.txt

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+CREATE FUNCTION getNthHighestSalary(@N INT) RETURNS INT AS
+BEGIN
+    DECLARE @res INT;
+    
+    WITH
+    ranks AS
+        (SELECT
+            Salary, 
+            DENSE_RANK() OVER (ORDER BY Salary DESC) as rank
+        FROM Employee),
+    result AS
+        (SELECT Salary AS SecondHighestSalary
+        FROM ranks
+        WHERE rank = @N)
+   SELECT @res = (SELECT ISNULL(
+            (SELECT TOP 1 SecondHighestSalary FROM result),
+            NULL) AS SecondHighestSalary);
+
+    RETURN(@res);
+END

database/178_Rank_Scores.txt

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+SELECT
+    score, 
+    DENSE_RANK() OVER (ORDER BY score DESC) as rank
+FROM scores

database/180_Consecutive_Numbers.txt

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+WITH
+LogsOrdered AS
+(SELECT
+    Num,
+    LAG(Num,1) OVER (ORDER BY Id) AS NumLag1,
+    LAG(Num,2) OVER (ORDER BY Id) AS NumLag2
+FROM Logs)
+SELECT DISTINCT Num AS ConsecutiveNums
+FROM LogsOrdered
+WHERE 
+    Num = NumLag1 
+    AND Num = NumLag2

database/181_Employees_Earning_More_Than_Their_Managers.txt

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+SELECT e.Name AS Employee 
+FROM Employee AS e
+LEFT JOIN Employee AS m 
+    ON e.ManagerId = m.Id
+WHERE
+    e.Salary > m.Salary

database/182_Duplicate_Emails.txt

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+WITH
+counts AS
+(SELECT
+    Email,
+    COUNT(*) AS cnt
+FROM Person
+GROUP BY Email)
+SELECT Email
+FROM counts
+WHERE cnt >= 2

database/183_Customers_Who_Never_Order.txt

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+WITH customerIds AS
+(SELECT DISTINCT c.ID
+FROM Customers AS c
+LEFT JOIN Orders AS o 
+    ON c.Id = o.CustomerId
+WHERE
+    o.Id IS NULL)
+SELECT Name AS Customers
+FROM Customers
+WHERE Id IN (SELECT Id From customerIds)

database/184_Department_Highest_Salary.txt

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+WITH 
+ranks AS(
+    SELECT
+        d.Name AS Department, 
+        e.Name AS Employee, 
+        e.Salary AS Salary,
+        RANK() OVER (PARTITION BY d.Id ORDER BY e.Salary DESC) AS rank
+    FROM Employee AS e
+    LEFT JOIN Department AS d
+        ON e.DepartmentId = d.Id)
+SELECT 
+    Department, 
+    Employee,
+    Salary
+FROM ranks
+WHERE
+    rank = 1
+    AND Department IS NOT NULL