Added database problems

LeetcodeProblems/Databases/Combine_Two_Tables.sql

@@ -0,0 +1,33 @@
+/*
+Combine Two Tables
+
+Table: Person
+
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| PersonId    | int     |
+| FirstName   | varchar |
+| LastName    | varchar |
++-------------+---------+
+PersonId is the primary key column for this table.
+Table: Address
+
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| AddressId   | int     |
+| PersonId    | int     |
+| City        | varchar |
+| State       | varchar |
++-------------+---------+
+AddressId is the primary key column for this table.
+ 
+
+Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those people:
+
+FirstName, LastName, City, State
+*/
+
+SELECT Person.FirstName, Person.LastName, Address.City, Address.State
+FROM Person LEFT JOIN Address ON Person.personID = Address.PersonID;

LeetcodeProblems/Databases/Second_highest_salary.sql

@@ -0,0 +1,29 @@
+/*
+Second Highest Salary
+
+Write a SQL query to get the second highest salary from the Employee table.
+
++----+--------+
+| Id | Salary |
++----+--------+
+| 1  | 100    |
+| 2  | 200    |
+| 3  | 300    |
++----+--------+
+For example, given the above Employee table, the query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.
+
++---------------------+
+| SecondHighestSalary |
++---------------------+
+| 200                 |
++---------------------+
+*/
+
+SELECT IFNULL(
+    (
+        SELECT DISTINCT(Salary)
+        FROM employee
+        ORDER BY employee.Salary DESC
+        LIMIT 1
+        OFFSET 1    
+    ), NULL) AS "SecondHighestSalary"

LeetcodeProblems/Databases/nth_Highest_Salary.sql

@@ -0,0 +1,34 @@
+/*
+Nth Highest Salary
+
+https://leetcode.com/problems/nth-highest-salary/submissions/
+
+Write a SQL query to get the nth highest salary from the Employee table.
+
++----+--------+
+| Id | Salary |
++----+--------+
+| 1  | 100    |
+| 2  | 200    |
+| 3  | 300    |
++----+--------+
+For example, given the above Employee table, the nth highest salary where n = 2 is 200. If there is no nth highest salary, then the query should return null.
+
++------------------------+
+| getNthHighestSalary(2) |
++------------------------+
+| 200                    |
++------------------------+
+*/
+CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
+BEGIN
+  DECLARE n_offset INT;
+  SET n_offset = N -1;
+  RETURN (
+      SELECT DISTINCT(Salary)
+      FROM employee
+      ORDER BY Salary DESC 
+      LIMIT 1
+      OFFSET n_offset
+  );
+END

README.md

@@ -76,7 +76,14 @@ To run a specific problem in your console, go to the file test, add the call to
 ### Sorting Algorithms
 | Algoritmhs |
 |      -     |
-| [Heap Sort](/SortingAlgorithms/heapSort.js)  | 
+| [Heap Sort](/SortingAlgorithms/heapSort.js)  |
+
+### Databases
+| Problems | Level | Link |
+|-|-|-|
+| [Combine Two Tables](/LeetcodeProblems/Databases/Combine_Two_Tables.sql) | Easy   | https://leetcode.com/problems/combine-two-tables |
+| [Second Highest Salary](/LeetcodeProblems/Databases/Second_highest_salary.sql)| Easy   | https://leetcode.com/problems/second-highest-salary |
+| [Nth Highest Salary](/LeetcodeProblems/Databases/nth_Highest_Salary.sql) | Medium | https://leetcode.com/problems/nth-highest-salary |
 
 
 ### UtilsClasses