Hp Integrity NonStop J-Series Manuel d'utilisateur télécharger pdf (Page 239)

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Sample Project-Tracking System

Identifying Key Fields

In the "events" and "respfor" files, however, neither a

"proj-code" nor an "event-num" alone is sufficient to

identify a record uniquely. Each record in those files can be

uniquely identified only by a combination of both "proj-code" and

"event-num." Therefore, the primary key for these files must be

a composite key group consisting of both "proj-code" and

"event-num."

In addition to a primary key for each file, you may want to

identify one or more alternate keys. The value of an alternate

key field need not uniquely identify a record, but instead,

provides an access path sorted in an alternate order. For

example, while the "proj-desc" field of the "projects" file may

have duplicate values, you can use this field as an alternate key

field to display all projects with the same description.

When a key from one file appears as a field in a related file,

you should make that field an alternate key in the related file

as well. This provides you with the means to link the files

within a multifile application.

Figure 12-8 shows a chart that illustrates the possible

relationships between the files and keys used by the project-

tracking system.

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Hp Integrity NonStop J-Series Manuel d'utilisateur Page 239

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