updated docs

Author: mahmoudparsian

code/bonus_chapters/join/README.md

@@ -1,5 +1,26 @@
 # Join operation
 
+------
+
+* In a nutshell, a join is an SQL operation performed 
+to establish a connection between two (or more) database 
+tables based on matching columns, thereby creating a 
+relationship between the tables. 
+
+* Types of joins
+	* Cross join. A cross join returns all possible combinations 
+	  of rows of two tables (also called a Cartesian product).
+
+	* Join/inner join. An inner join, also known as a simple join, 
+	  returns rows from joined tables that have matching rows. 
+
+	* Left outer join/left join.
+
+	* Right outer join/right join.
+
+	* Full outer join.
+
+-----------
 
 ![joins](../../../images/sql_joins.png)