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7. Database System Architecture > 7.2. Database Systems

Database Systems

A database system is often pictured as a three-level structure, as shown in Figure 7-1.

The three-level structure of a database system
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 has no logical meaning, as related to the database. However, the data does have a very definite physical structure to allow efficient access. In other words, the data is 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, I 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 .


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