iX V6.5 (30391-90011)

Chapter 2 35

Database Structure and Protection

Database Elements

2 Database Structure and Protection

This chapter describes the structure of a TurboIMAGE/XL database. The discussion

deﬁnes the elements of a database and describes how they are related and how they can be

accessed. If you are designing a TurboIMAGE/XL database, use this chapter with chapter

3 which describes how to deﬁne a database.

Database Elements

A database is a named collection of related data. The formal description of this data is

called a schema. The database is deﬁned in terms of data items, data entries, and data

sets which are described in the text below.

Data Items

A data item is the smallest accessible data element in a database. Each data item consists

of a value referenced by a data item name. The name is usually selected to describe the

data value. Many data item values can be referenced by the same data item name with

each value existing in a different data entry.

A compound data item is a named group of identically deﬁned, adjacent items within

the same data entry. Each occurrence of the data item is called a sub-item, and each

sub-item can have a value. A compound item is similar to an array in programming

languages such as FORTRAN 77 and BBASIC. For example, a data entry might contain a

compound item named MONTHLY-SALES with 12 sub-items in which the total sales for each

month are recorded.

Critical items are deﬁned as the key item in a master data set and the search and sort

items in detail data sets. These are described later in this chapter.

A data item type can be one of several types of integers, real or ﬂoating-point numbers, or

ASCII character information. The database designer deﬁnes each data item as a particular

type depending on what kind of information is to be stored in the item. The data types are

described in detail in the next chapter and are summarized in Tables 3-2 and 3-3.

Data Entries

A data entry is an ordered set of related data items (sometimes referred to as a record).

Specify the order of data items in an entry when you deﬁne the database. Data entries can

be deﬁned with at most 255 data item names; none can be repeated. The length of the data

entry is the combined length of the data items it contains and cannot exceed 2348

half-words or 4696 bytes.