Logical Database Design

Описание: Before designing a database, you need to know the two ways information is viewed in a database. The physical view involves how data is stored on and retrieved from storage media, such as hard disks, magnetic tapes, or CDs. For each database, there is only one physical view of data. The logical view involves how information appears to users and how it can be organized and retrieved.

The first step in database design is defining a data model, which determines how data is created, represented, organized, and maintained.

A data model usually includes these three components. Data structure describes how data is organized and the relationship among records. Operations describes methods, calculations, and so forth that can be performed on data, such as updating and querying data. Integrity rules define the boundaries of a database, such as maximum and minimum values allowed for a field, constraints (limits on what type of data can be stored in a field), and access methods.

In a hierarchical model, the relationships among records form a treelike structure. Records are called nodes, and relationships among records are called branches. The node at the top is called the root, and every other node (called a child) has a parent. Nodes with the same parents are called twins or siblings.

The network model is similar to the hierarchical model, but records are organized differently. This model links invoice number, customer number, and method of payment. A relational model uses a two-dimensional table of rows and columns of data. Rows are records (also called tuples), and columns are fields (also referred to as attributes).