I got this error message when migrating my Magento files and database from one server to another.
Notice: Undefined index: 0 in app/code/core/Mage/Core/Model/Mysql4/Config.php on line 92
Solution:

The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables. [@via w3schools]
A SQL JOIN clause combines records from two or more tables in a database. It creates a set that can be saved as a table or used as is. A JOIN is a means for combining fields from two tables by using values common to each. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. In special cases, …

Suppose, you have a module called MyModule. Its version is 0.1.0. Now, you want to do some database changes for the module. You have the mysql setup file (mysql install file) mysql4-install-0.1.0.php in MyModule/sql/mymodule_setup folder of your module.
You don’t need to make direct changes to database. You can upgrade your module to make your necessary database changes. To do so,

Difference between EAV and Flat Catalog
In EAV database model, data are stored in different smaller tables rather than storing in a single table.
Like,
product name is stored in catalog_product_entity_varchar table
product id is stored in catalog_product_entity_int table
product price is stored in catalog_product_entity_decimal table

For using database, you need to install a package of PEAR called ‘MDB2’ along with the installation of PEAR and Smarty.
MDB2 provides a common API for all support RDBMS.

1) To make a backup of a single MySQL database
You have to make backup before you enter inside your MySQL system i.e. by typing mysql –u username –p password . (Here, username and password indicates your MySQL server username and password.)

Problem:
“Fails to start the service. Please wait 30 seconds to try again.”
I faced this problem 2 years back when I was a newbie to MySQL. I was just starting to learn PHP and MySQL at that time.