Understanding Database Software

To understand how a database program works, you first need to have an understanding of certain terms. Some of the terms you are already familiar with, whereas others you may not be. These include database, record, field, pointer, index, primary key, and secondary key. They are defined as follows:

. Database\ A database holds information that is related to a specific type of application-payroll, personnel, supply inventory, and so on. In this context, the term database is often considered synonymous with file. This is especially true when dealing with database files. l Record\ A record consists of a group of related fields, all pertaining to the same subject: a person, a thing, or an event.

Figure 3-6.\Example of a database report.

Field\ A field consists of one unit of information. A field is also referred to as a data item or a data element. It maybe alphabetic, like your name (John or Jane Doe); numeric, like your ZIP Code (01234); alphanumeric, like your post office box or street address (P. O. Box 669 or 1234 Main St.); or logical (true/false), like on leave (true-on leave, false-not on leave).

Pointer\ A pointer is a data item in one record that identifies the storage location of another logically related record.

Index\ An index enables you to access records in a database (also referred to as database file or file) in the order of the index regardless of the physical sequence of the records in the database. You can think of indexing as sorting without having to sort. The index itself is a file. It contains a duplicate of the key field (or fields) such as account number, or name and security number, and a pointer to the actual disk record identified with this key in another permanent disk file. For example, if there are 5,000 records in your database, and the key field happens to be SSN, then the index would also contain 5,000 entries with each entry having an SSN. It is also possible for you to have one or more secondary indexes that contain other various secondary key fields.

Primary key\ The primary key in a database consists of a unique identifier for a particular record and should only point to a single record in the database being indexed.

Secondary key\ Data are normally arranged within a database in some type of order, depending upon the contents of one or more fields. Secondary keys allow you to access the database in different ways. For example, your database might be arranged in the order the records were entered. You can then set up a secondary index (or key) by the name field, or by the social security number field. You may specify any number of secondary keys. You might index by more than one field. For example, you could index by last name within rate, as shown in figure 3-6.

Database Organization Methods/Structures

Databases can be list, hierarchical, network, or relational in structure. The major advantage of a

database is it permits the maintenance of a related set of files or tables that can provide information to several different users. So how do these database structures differ? you might ask. That's a good question. Read on and find out.

LIST DATABASES.\ List databases link records together through the use of pointers. The pointer is a data element in one record (normally the master record) that points to the actual disk location of another logically related record, as illustrated in figure 3-7.

HIERARCHICAL DATABASES.\ Hierarchical databases consist of elements that act in a superior-subordinate or parent-child relationship. What this means is that one element is linked to another element in the database. The superior element points to one or more subordinate elements. There can also be a subordinate of a subordinate, which will enable many hundreds of elements to be connected. See figure 3-8.