Feat: add database solutions

Author: remy727

.gitignore

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+.DS_Store

README.md

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 | 2409 | Count Days Spent Together | [Ruby](./algorithms/ruby/2409-count-days-spent-together.rb) | Easy |
 | 2413 | Smallest Even Multiple | [Ruby](./algorithms/ruby/2413-smallest-even-multiple.rb) | Easy |
 
+### Database
+| # | Title | Solution | Difficulty |
+|---| ----- | -------- | ---------- |
+| 175 | Combine Two Tables | [Database](./database/0175-combine-two-tables.sql) | Easy |
+| 181 | Employees Earning More Than Their Managers | [Database](./database/0181-employees-earning-more-than-their-managers.sql) | Easy |
+| 182 | Duplicate Emails | [Database](./database/0182-duplicate-emails.sql) | Easy |
+| 183 | Customers Who Never Order | [Database](./database/0183-customers-who-never-order.sql) | Easy |
+| 196 | Delete Duplicate Emails | [Database](./database/0196-delete-duplicate-emails.sql) | Easy |
+| 197 | Rising Temperature | [Database](./database/0197-rising-temperature.sql) | Easy |
+| 584 | Find Customer Referee | [Database](./database/0584-find-customer-referee.sql) | Easy |
+
 ### Shell
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |

database/0175-combine-two-tables.sql

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+-- Problem: 175. Combine Two Tables
+-- URL: https://leetcode.com/problems/combine-two-tables
+
+-- Write your MySQL query statement below
+SELECT
+  firstName,
+  lastName,
+  city,
+  state
+FROM
+  Person
+LEFT JOIN Address on Person.personId = Address.personId
+;

database/0181-employees-earning-more-than-their-managers.sql

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+-- Problem: 181. Employees Earning More Than Their Managers
+-- URL: https://leetcode.com/problems/employees-earning-more-than-their-managers
+
+-- Write your MySQL query statement below
+SELECT
+  a.name AS Employee
+FROM
+  Employee AS a
+JOIN Employee AS b ON a.managerId = b.id AND a.salary > b.salary
+;

database/0182-duplicate-emails.sql

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+-- Problem: 182. Duplicate Emails
+-- URL: https://leetcode.com/problems/duplicate-emails
+
+-- Write your MySQL query statement below
+SELECT
+	email
+FROM
+	Person
+group by
+	email
+having count(email) > 1
+;

database/0183-customers-who-never-order.sql

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+-- Problem: 183. Customers Who Never Order
+-- URL: https://leetcode.com/problems/customers-who-never-order
+
+-- Write your MySQL query statement below
+SELECT
+  name as 'Customers'
+FROM
+  customers
+WHERE
+  id NOT IN
+  (
+    SELECT customerid FROM Orders
+  )
+;

database/0196-delete-duplicate-emails.sql

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+-- Problem: 196. Delete Duplicate Emails
+-- URL: https://leetcode.com/problems/delete-duplicate-emails
+
+-- Please write a DELETE statement and DO NOT write a SELECT statement.
+-- Write your MySQL query statement below
+DELETE
+  p1
+FROM
+  Person p1, Person p2
+WHERE
+  p1.email = p2.email and p1.id > p2.id
+;

database/0197-rising-temperature.sql

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+-- Problem: 197. Rising Temperature
+-- URL: https://leetcode.com/problems/rising-temperature
+
+-- Write your MySQL query statement below
+SELECT
+  weather.id AS 'Id'
+FROM
+  weather
+JOIN
+  weather w ON DATEDIFF(weather.recordDate, w.recordDate) = 1
+  AND weather.temperature > w.temperature
+;

database/0584-find-customer-referee.sql

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+-- Problem: 584. Find Customer Referee
+-- URL: https://leetcode.com/problems/find-customer-referee
+
+-- Write your MySQL query statement below
+SELECT
+  name
+FROM
+  Customer
+WHERE
+  referee_id is NULL OR referee_id <> 2
+;