Thoughts of mine

MySQL LEFT, RIGHT JOIN

MySQL joins are hard for beginners. At least for me when I was beginner.
I will try to explain the joins in the simplest possible way.
Join in MySQL is a query where you can join one or more tables.
For example we have two tables: products and buyers with the following structures.
Table products:

mysql> SELECT * FROM products;
+----+--------------+--------------+
| id | product_name | manufacturer |
+----+--------------+--------------+
|  1 | Shoes        | Company1     |
|  2 | Laptop       | Company2     |
|  3 | Monitor      | Company3     |
|  4 | DVD          | Company4     |
+----+--------------+--------------+
4 rows in set (0.00 sec)

Table buyers:
mysql> SELECT * FROM buyers;

+----+------+------------+----------+
| id | pid  | buyer_name | quantity |
+----+------+------------+----------+
|  1 |    1 | Steve      |        2 |
|  2 |    2 | John       |        1 |
|  3 |    3 | Larry      |        1 |
|  4 |    3 | Michael    |        5 |
|  5 | NULL | Steven     |     NULL |
+----+------+------------+----------+
5 rows in set (0.00 sec)

Left Join

mysql> SELECT buyer_name, quantity, product_name FROM buyers LEFT JOIN products ON
 buyers.pid=products.id;
+------------+----------+--------------+
| buyer_name | quantity | product_name |
+------------+----------+--------------+
| Steve      |        2 | Shoes        |
| John       |        1 | Laptop       |
| Larry      |        1 | Monitor      |
| Michael    |        5 | Monitor      |
| Steven     |     NULL | NULL         |
+------------+----------+--------------+
5 rows in set (0.00 sec)

What happened?
Mysql starts with the left table (buyers). For each row from the table buyers mysql scans the table products, finds the id of the product and returns the product name. Then the product name is joined with the matching row from the table buyers. For unmatched rows it returns null.
To make it simpler, the above query is same as (except the unmatched rows are not returned):

mysql> SELECT buyers.buyer_name, buyers.quantity, products.product_name FROM buyer
s,products WHERE buyers.pid=products.id;
+------------+----------+--------------+
| buyer_name | quantity | product_name |
+------------+----------+--------------+
| Steve      |        2 | Shoes        |
| John       |        1 | Laptop       |
| Larry      |        1 | Monitor      |
| Michael    |        5 | Monitor      |
+------------+----------+--------------+
4 rows in set (0.00 sec)

Ruby On Rails 'Include' and 'Extend' example

Module level class method can't be available to any class use 'Include' Or 'Extend', Only modules instance methods available

When use include keyword for include a module into class it make module level instance method as to class level instance method