Access Database Design and Programming, Second Edition

Database Systems

A database system is often pictured as a three-level structure, as shown in Figure 7.1.

Figure 7-1. The three-level structure of a database system

At the lowest level of the structure is the physical database , which consists of the raw data existing on a physical object, such as a hard disk. At this level, the data have no logical meaning, as related to the database. However, the data do have a very definite physical structure, to allow efficient access. In other words, the data are more than just a string of bits.

In fact, there are a variety of structures in which the data might be stored, including hash tables, balanced trees, linked lists, nested records, and so on, and the choice of data structure is not a simple one. However, we will not pursue a discussion of the physical database in this book. Suffice it to say that, at the physical level, the data is viewed as a structured collection of bits and the sole purpose of the structure is to provide efficient access to the data. The physical level of a database is often referred to as the internal level .

The conceptual database is a conceptual view of the database as a whole. It gives the data a logical structure. For instance, in a relational database system, the data are viewed as a collection of tables, with column headings describing the attributes of the corresponding entity class. Moreover, tables are related ...