SQL Index establishes compliance with six major iron laws

Iron Law One: there is no free lunch, it takes a price to use an index.

Iron Law II: Do not index columns that are rarely involved in a query or that have more duplicate values.

Iron Law III: For columns that are queried by scope, it is best to index.

Iron Law four: if the table has a primary key or foreign key, it must be indexed.

Iron Law V: for some special data types, do not create an index.

Iron Law VI: An index can be integrated with a collection of Where statements.

SQL Index establishes compliance with six major iron laws