Databases
A database is an organized collection of information or data that can be displayed by a computer. It is a great form of record keeping. Databases use tables of data. Three different relationships exist among tables in databases.

One-to-one
A one-to-one relationship allows for one record on a table to be matched to another single record on a separate table. This type of relationship is commonly used when security issues are present.

One-to-many
A one-to-many relationship is the most common type of relationship used in databases. In this case, a record from one table can have multiple matches from another table. An example of this is a newspaper (one record), which has many subscribers (multiple records).

Many-to-many
This type of relationship is fairly common in professional databases. Each record on a table can have multiple matches on another table and vice-versa. In reality, many-to-many relationships can be seen as two one-to-many relationships with a table linking them together. An example of this is a newspaper that has many readers, and a reader who reads many newspapers.

Three-tier model
This model illustrates a special type of architecture where there are three separate processes being run.

• The first tier is the user interface. This can be the client software/web browser, and is what the user actually sees.
• The second tier is the business logic. This is the bridge between the client and the database.
• The third tier is the back-end. This is the actual database.

Transactions
Another important feature of databases are transactions. These can be described as a set of actions that must be completed on an all or nothing basis in order to be successful. Online banking is a perfect example of a transaction.