diff --git a/14. Databases/README.md b/14. Databases/README.md
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--- a/14. Databases/README.md	
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 # 14. Databases
 
+## Basic SQL
+
+```sql
+SELECT CITY FROM STATION WHERE CODE = '123'
+```
+
+Get distinct cities:
+
+```sql
+SELECT DISTINCT CITY FROM STATION WHERE CODE = '123'
+```
+
+Count number of distinct cities
+
+```sql
+SELECT count(DISTINCT CITY) FROM STATION WHERE CODE = '123'
+```
+
+Apart from count, there is also `sum`, `avg`, `ceil`, `floor`.
+
+Get the shortest city name and longest city name
+
+```sql
+SELECT CITY, LENGTH(CITY) FROM STATION
+ORDER BY LENGTH(CITY), CITY asc 
+limit 1;
+
+SELECT CITY, LENGTH(CITY) FROM STATION
+ORDER BY LENGTH(CITY), CITY desc 
+limit 1;
+
+```
+
+Get cities ending with vowels
+
+```sql
+SELECT DISTINCT CITY FROM STATION WHERE city REGEXP "[aeiou]$"
+```
+
+Select substring of column. The first two characters. 1 means first character, 2 means length. SQL has an index system starting from 1. To get the last ones, use -X.
+
+```sql
+SELECT SUBSTR(NAME, 1, 2) FROM TABLE
+```
+
+Output a string for each case
+
+```sql
+SELECT
+    CASE
+        WHEN A >= (B + C) OR B >= (A + C) OR C >= (A + B) THEN 'Not A Triangle'
+        WHEN A = B AND A = C THEN 'Equilateral'
+        WHEN A = B OR B = C OR A = C THEN 'Isosceles'
+        ELSE 'Scalene'
+    END
+FROM TRIANGLES;
+```
+
+Get the list of top earning employees, and the number of employees that earn this amount
+
+```sql
+SELECT earnings, count(*) FROM Employee
+GROUP BY earnings
+ORDER BY earnings desc
+limit 1;
+```
+
+```sql
+SELECT COUNTRY.Continent, FLOOR(AVG(CITY.Population)) 
+FROM CITY INNER JOIN COUNTRY 
+ON CITY.CountryCode=COUNTRY.Code
+GROUP BY COUNTRY.Continent;
+```
+
+## Book SQL
+
 ```sql
 SELECT CourseName, TeacherName FROM Courses, Teachers WHERE Courses.TeacherID = Teachers.TeacherID
 ```
diff --git a/14. Databases/cheatsheet.md b/14. Databases/cheatsheet.md
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+# [Github cheatsheet](https://github.com/khanhnamle1994/cracking-the-data-science-interview/tree/master/Cheatsheets#sql)
+
+## SQL
+
+* [Finding Data Queries](#finding-data-queries)
+* [Data Modification Queries](#data-modification-queries)
+* [Reporting Queries](#reporting-queries)
+* [Join Queries](#join-queries)
+* [View Queries](#view-queries)
+* [Altering Table Queries](#altering-table-queries)
+* [Creating Table Query](#creating-table-query)
+
+### Finding Data Queries
+
+#### **SELECT**: used to select data from a database
+* `SELECT` * `FROM` table_name;
+
+#### **DISTINCT**: filters away duplicate values and returns rows of specified column
+* `SELECT DISTINCT` column_name;
+
+#### **WHERE**: used to filter records/rows
+* `SELECT` column1, column2 `FROM` table_name `WHERE` condition;
+* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` condition2;
+* `SELECT` * `FROM` table_name `WHERE` condition1 `OR` condition2;
+* `SELECT` * `FROM` table_name `WHERE NOT` condition;
+* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` (condition2 `OR` condition3);
+* `SELECT` * `FROM` table_name `WHERE EXISTS` (`SELECT` column_name `FROM` table_name `WHERE` condition);
+
+#### **ORDER BY**: used to sort the result-set in ascending or descending order
+* `SELECT` * `FROM` table_name `ORDER BY` column;
+* `SELECT` * `FROM` table_name `ORDER BY` column `DESC`;
+* `SELECT` * `FROM` table_name `ORDER BY` column1 `ASC`, column2 `DESC`;
+
+#### **SELECT TOP**: used to specify the number of records to return from top of table
+* `SELECT TOP` number columns_names `FROM` table_name `WHERE` condition;
+* `SELECT TOP` percent columns_names `FROM` table_name `WHERE` condition;
+* Not all database systems support `SELECT TOP`. The MySQL equivalent is the `LIMIT` clause
+* `SELECT` column_names `FROM` table_name `LIMIT` offset, count;
+
+#### **LIKE**: operator used in a WHERE clause to search for a specific pattern in a column
+* % (percent sign) is a wildcard character that represents zero, one, or multiple characters
+* _ (underscore) is a wildcard character that represents a single character
+* `SELECT` column_names `FROM` table_name `WHERE` column_name `LIKE` pattern;
+* `LIKE` ‘a%’ (find any values that start with “a”)
+* `LIKE` ‘%a’ (find any values that end with “a”)
+* `LIKE` ‘%or%’ (find any values that have “or” in any position)
+* `LIKE` ‘[a-c]%’ (find any values starting with “a”, “b”, or “c”
+
+#### **IN**: operator that allows you to specify multiple values in a WHERE clause
+* essentially the IN operator is shorthand for multiple OR conditions
+* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (value1, value2, …);
+* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (`SELECT STATEMENT`);
+
+#### **BETWEEN**: operator selects values within a given range inclusive
+* `SELECT` column_names `FROM` table_name `WHERE` column_name `BETWEEN` value1 `AND` value2;
+* `SELECT` * `FROM` Products `WHERE` (column_name `BETWEEN` value1 `AND` value2) `AND NOT` column_name2 `IN` (value3, value4);
+* `SELECT` * `FROM` Products `WHERE` column_name `BETWEEN` #01/07/1999# AND #03/12/1999#;
+
+#### **NULL**: values in a field with no value
+* `SELECT` * `FROM` table_name `WHERE` column_name `IS NULL`;
+* `SELECT` * `FROM` table_name `WHERE` column_name `IS NOT NULL`;
+
+#### **AS**: aliases are used to assign a temporary name to a table or column
+* `SELECT` column_name `AS` alias_name `FROM` table_name;
+* `SELECT` column_name `FROM` table_name `AS` alias_name;
+* `SELECT` column_name `AS` alias_name1, column_name2 `AS` alias_name2;
+* `SELECT` column_name1, column_name2 + ‘, ‘ + column_name3 `AS` alias_name;
+
+#### **UNION**: set operator used to combine the result-set of two or more SELECT statements
+* Each SELECT statement within UNION must have the same number of columns
+* The columns must have similar data types
+* The columns in each SELECT statement must also be in the same order
+* `SELECT` columns_names `FROM` table1 `UNION SELECT` column_name `FROM` table2;
+* `UNION` operator only selects distinct values, `UNION ALL` will allow duplicates
+
+#### **INTERSECT**: set operator which is used to return the records that two SELECT statements have in common
+* Generally used the same way as **UNION** above
+* `SELECT` columns_names `FROM` table1 `INTERSECT SELECT` column_name `FROM` table2;
+
+#### **EXCEPT**: set operator used to return all the records in the first SELECT statement that are not found in the second SELECT statement
+* Generally used the same way as **UNION** above
+* `SELECT` columns_names `FROM` table1 `EXCEPT SELECT` column_name `FROM` table2;
+
+#### **ANY|ALL**: operator used to check subquery conditions used within a WHERE or HAVING clauses
+* The `ANY` operator returns true if any subquery values meet the condition
+* The `ALL` operator returns true if all subquery values meet the condition
+* `SELECT` columns_names `FROM` table1 `WHERE` column_name operator (`ANY`|`ALL`) (`SELECT` column_name `FROM` table_name `WHERE` condition);
+
+#### **GROUP BY**: statement often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns
+* `SELECT` column_name1, COUNT(column_name2) `FROM` table_name `WHERE` condition `GROUP BY` column_name1 `ORDER BY` COUNT(column_name2) DESC;
+
+#### **HAVING**: this clause was added to SQL because the WHERE keyword could not be used with aggregate functions
+* `SELECT` `COUNT`(column_name1), column_name2 `FROM` table `GROUP BY` column_name2 `HAVING` `COUNT(`column_name1`)` > 5;
+
+### Data Modification Queries
+
+#### **INSERT INTO**: used to insert new records/rows in a table
+* `INSERT INTO` table_name (column1, column2) `VALUES` (value1, value2);
+* `INSERT INTO` table_name `VALUES` (value1, value2 …);
+
+#### **UPDATE**: used to modify the existing records in a table
+* `UPDATE` table_name `SET` column1 = value1, column2 = value2 `WHERE` condition;
+* `UPDATE` table_name `SET` column_name = value;
+
+#### **DELETE**: used to delete existing records/rows in a table
+* `DELETE FROM` table_name `WHERE` condition;
+* `DELETE` * `FROM` table_name;
+
+### Reporting Queries
+
+#### **COUNT**: returns the # of occurrences
+* `SELECT COUNT (DISTINCT` column_name`)`;
+
+#### **MIN() and MAX()**: returns the smallest/largest value of the selected column
+* `SELECT MIN (`column_names`) FROM` table_name `WHERE` condition;
+* `SELECT MAX (`column_names`) FROM` table_name `WHERE` condition;
+
+#### **AVG()**: returns the average value of a numeric column
+* `SELECT AVG (`column_name`) FROM` table_name `WHERE` condition;
+
+#### **SUM()**: returns the total sum of a numeric column
+* `SELECT SUM (`column_name`) FROM` table_name `WHERE` condition;
+
+### Join Queries
+
+####  **INNER JOIN**: returns records that have matching value in both tables
+* `SELECT` column_names `FROM` table1 `INNER JOIN` table2 `ON` table1.column_name=table2.column_name;
+* `SELECT` table1.column_name1, table2.column_name2, table3.column_name3 `FROM` ((table1 `INNER JOIN` table2 `ON` relationship) `INNER JOIN` table3 `ON` relationship);
+
+#### **LEFT (OUTER) JOIN**: returns all records from the left table (table1), and the matched records from the right table (table2)
+* `SELECT` column_names `FROM` table1 `LEFT JOIN` table2 `ON` table1.column_name=table2.column_name;
+
+### **RIGHT (OUTER) JOIN**: returns all records from the right table (table2), and the matched records from the left table (table1)
+* `SELECT` column_names `FROM` table1 `RIGHT JOIN` table2 `ON` table1.column_name=table2.column_name;
+
+#### **FULL (OUTER) JOIN**: returns all records when there is a match in either left or right table
+* `SELECT` column_names `FROM` table1 ``FULL OUTER JOIN`` table2 `ON` table1.column_name=table2.column_name;
+
+#### **Self JOIN**: a regular join, but the table is joined with itself
+* `SELECT` column_names `FROM` table1 T1, table1 T2 `WHERE` condition;
+
+### View Queries
+
+#### **CREATE**: create a view
+* `CREATE VIEW` view_name `AS SELECT` column1, column2 `FROM` table_name `WHERE` condition;
+
+#### **SELECT**: retrieve a view
+* `SELECT` * `FROM` view_name;
+
+#### **DROP**: drop a view
+* `DROP VIEW` view_name;
+
+### Altering Table Queries
+
+#### **ADD**: add a column
+* `ALTER TABLE` table_name `ADD` column_name column_definition;
+
+#### **MODIFY**: change data type of column
+* `ALTER TABLE` table_name `MODIFY` column_name column_type;
+
+#### **DROP**: delete a column
+* `ALTER TABLE` table_name `DROP COLUMN` column_name;
+
+### Creating Table Query
+
+### **CREATE**: create a table
+* `CREATE TABLE` table_name `(` <br />
+   `column1` `datatype`, <br />
+   `column2` `datatype`, <br />
+   `column3` `datatype`, <br />
+   `column4` `datatype`, <br />
+   `);`
+
+