Added 5 solutions

Author: mrinal1704

Easy/Customers who never order.sql

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+-- Question 13
+-- Suppose that a website contains two tables, 
+-- the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.
+
+-- Table: Customers.
+
+-- +----+-------+
+-- | Id | Name  |
+-- +----+-------+
+-- | 1  | Joe   |
+-- | 2  | Henry |
+-- | 3  | Sam   |
+-- | 4  | Max   |
+-- +----+-------+
+-- Table: Orders.
+
+-- +----+------------+
+-- | Id | CustomerId |
+-- +----+------------+
+-- | 1  | 3          |
+-- | 2  | 1          |
+-- +----+------------+
+-- Using the above tables as example, return the following:
+
+-- +-----------+
+-- | Customers |
+-- +-----------+
+-- | Henry     |
+-- | Max       |
+-- +-----------+
+
+
+-- Solution
+Select Name as Customers
+from Customers
+where id != All(select c.id
+                from Customers c, Orders o
+                where c.id = o.Customerid) 

Easy/Department top three salaries.sql

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+-- Question 14
+-- The Employee table holds all employees. Every employee has an Id, and there is also a column for the department Id.
+
+-- +----+-------+--------+--------------+
+-- | Id | Name  | Salary | DepartmentId |
+-- +----+-------+--------+--------------+
+-- | 1  | Joe   | 85000  | 1            |
+-- | 2  | Henry | 80000  | 2            |
+-- | 3  | Sam   | 60000  | 2            |
+-- | 4  | Max   | 90000  | 1            |
+-- | 5  | Janet | 69000  | 1            |
+-- | 6  | Randy | 85000  | 1            |
+-- | 7  | Will  | 70000  | 1            |
+-- +----+-------+--------+--------------+
+-- The Department table holds all departments of the company.
+
+-- +----+----------+
+-- | Id | Name     |
+-- +----+----------+
+-- | 1  | IT       |
+-- | 2  | Sales    |
+-- +----+----------+
+-- Write a SQL query to find employees who earn the top three salaries in each of the department. For the above tables, your SQL query should return the following rows (order of rows does not matter).
+
+-- +------------+----------+--------+
+-- | Department | Employee | Salary |
+-- +------------+----------+--------+
+-- | IT         | Max      | 90000  |
+-- | IT         | Randy    | 85000  |
+-- | IT         | Joe      | 85000  |
+-- | IT         | Will     | 70000  |
+-- | Sales      | Henry    | 80000  |
+-- | Sales      | Sam      | 60000  |
+-- +------------+----------+--------+
+-- Explanation:
+
+-- In IT department, Max earns the highest salary, both Randy and Joe earn the second highest salary, 
+-- and Will earns the third highest salary. 
+-- There are only two employees in the Sales department, 
+-- Henry earns the highest salary while Sam earns the second highest salary.
+
+-- Solution
+select a.department, a.employee, a.salary
+from (
+select d.name as department, e.name as employee, salary, 
+    dense_rank() over(Partition by d.name order by salary desc) as rk
+from Employee e join Department d
+on e.departmentid = d.id) a
+where a.rk<4

Easy/Duplicate Emails.sql

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+-- Question 11
+-- Write a SQL query to find all duplicate emails in a table named Person.
+
+-- +----+---------+
+-- | Id | Email   |
+-- +----+---------+
+-- | 1  | [email protected] |
+-- | 2  | [email protected] |
+-- | 3  | [email protected] |
+-- +----+---------+
+-- For example, your query should return the following for the above table:
+
+-- +---------+
+-- | Email   |
+-- +---------+
+-- | [email protected] |
+-- +---------+
+
+
+-- Solution
+Select Email
+from
+(Select Email, count(Email)
+from person
+group by Email
+having count(Email)>1) a

Easy/Employees earning more than their managers.sql

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+-- Question 15
+-- The Employee table holds all employees including their managers. 
+-- Every employee has an Id, and there is also a column for the manager Id.
+
+-- +----+-------+--------+-----------+
+-- | Id | Name  | Salary | ManagerId |
+-- +----+-------+--------+-----------+
+-- | 1  | Joe   | 70000  | 3         |
+-- | 2  | Henry | 80000  | 4         |
+-- | 3  | Sam   | 60000  | NULL      |
+-- | 4  | Max   | 90000  | NULL      |
+-- +----+-------+--------+-----------+
+-- Given the Employee table, write a SQL query that finds out employees who earn more than their managers. 
+-- For the above table, Joe is the only employee who earns more than his manager.
+
+-- +----------+
+-- | Employee |
+-- +----------+
+-- | Joe      |
+-- +----------+
+
+-- Solution
+select a.Name as Employee
+from employee a, employee b
+where a.salary>b.salary and a.managerid=b.id

Easy/Rising Temperature.sql

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+-- Question 12
+-- Given a Weather table, write a SQL query to find all dates' Ids with higher temperature compared to its previous (yesterday's) dates.
+
+-- +---------+------------------+------------------+
+-- | Id(INT) | RecordDate(DATE) | Temperature(INT) |
+-- +---------+------------------+------------------+
+-- |       1 |       2015-01-01 |               10 |
+-- |       2 |       2015-01-02 |               25 |
+-- |       3 |       2015-01-03 |               20 |
+-- |       4 |       2015-01-04 |               30 |
+-- +---------+------------------+------------------+
+-- For example, return the following Ids for the above Weather table:
+
+-- +----+
+-- | Id |
+-- +----+
+-- |  2 |
+-- |  4 |
+-- +----+
+
+-- Solution
+select a.Id
+from weather a, weather b
+where a.Temperature>b.Temperature and  datediff(a.recorddate,b.recorddate)=1