HP Integrity NonStop J-Series - Manual (Page 49)

TASK OVERVIEW
Key Fields
S5044-018
Record 1
Record 2
Record 3
Record 4
Record n
Employee File
Field
Name:
Empnum
Field
Value
Empnum
0001
0002
0003
0004
• • •
Empname
Field
Value
Empname
Jack Jones
Marge Martin
Phil Smith
Mark Monte
• • •
Regnum
Field
Value
Regnum
01
01
02
01
• • •
Branchnum
Field
Value
Branchnum
01
02
03
02
• • •
Job
Field
Value
Job
Manager
Clerk
Clerk
Manager
• • •
• • •
Field
Values
• • •
• • •
• • •
• • •
• • •
• • •
Figure 2-13. Sample Employee File
Notice that this file resembles a table. Tandem computer systems
store information in relational data base form. With a
relational data base, you can regard each file as a
two-dimensional table, where the columns correspond to fields,
and the rows correspond to records.
Key Fields
A key field identifies a specific record within a file.
Sometimes more than one field is needed to identify a record.
When two or more contiguous fields are used to identify a
specific record, the combined fields are called a composite key
field.
A primary key field uniquely identifies a particular record.
Only one primary key field can exist for a record. A likely
primary key field for an employee record is empnum because only
one employee can have this unique number.
An alternate key field identifies records with a common property.
A record can have more than one alternate key. When a record has
an alternate key, the application can use alternate search
paths to retrieve data.
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