You can use more than one key field to arrange the records in a
database file. The key fields do not have to use the same sequence.
For example, when you use two key fields, one field can use ascending sequence
while the other can use descending sequence. Consider the following records.
| Record |
Order |
Ordate |
Line |
Item |
Qtyord |
Extens |
|---|
| 1 |
52218 |
063088 |
01 |
88682 |
425 |
031875 |
| 2 |
41834 |
062888 |
03 |
42111 |
30 |
020550 |
| 3 |
41834 |
062888 |
02 |
61132 |
4 |
021700 |
| 4 |
52218 |
063088 |
02 |
40001 |
62 |
021700 |
| 5 |
41834 |
062888 |
01 |
00623 |
50 |
025000 |
|
If the access path uses the Order field, then the Line field
as the key fields, both in ascending sequence, the order of the records in
the access path is.
| Record |
Order |
Ordate |
Line |
Item |
Qtyord |
Extens |
|---|
| 5 |
41834 |
062888 |
01 |
00623 |
50 |
025000 |
| 3 |
41834 |
062888 |
02 |
61132 |
4 |
021700 |
| 2 |
41834 |
062888 |
03 |
42111 |
30 |
020550 |
| 1 |
52218 |
063088 |
01 |
88682 |
425 |
031875 |
| 4 |
52218 |
063088 |
02 |
40001 |
62 |
021700 |
|
If the access path uses the key field Order in ascending sequence,
then the Line field in descending sequence, the order of the records
in the access path is.
| Record |
Order |
Ordate |
Line |
Item |
Qtyord |
Extens |
|---|
| 2 |
41834 |
062888 |
03 |
42111 |
30 |
020550 |
| 3 |
41834 |
062888 |
02 |
61132 |
4 |
021700 |
| 5 |
41834 |
062888 |
01 |
00623 |
50 |
025000 |
| 4 |
52218 |
063088 |
02 |
40001 |
62 |
021700 |
| 1 |
52218 |
063088 |
01 |
88682 |
425 |
031875 |
|
When a record has key fields whose contents are the same as the key field
in another record in the same file, then the file is said to have records
with duplicate key values. However, the duplication must occur for all key
fields for a record if they are to be called duplicate key values. For example,
if a record format has two key fields Order and Ordate, duplicate
key values occur when the contents of both the Order and Ordate fields
are the same in two or more records. These records have duplicate key values.
| Order |
Ordate |
Line |
Item |
Qtyord |
Extens |
|---|
| 41834 |
062888 |
03 |
42111 |
30 |
020550 |
| 41834 |
062888 |
02 |
61132 |
04 |
021700 |
| 41834 |
062888 |
01 |
00623 |
50 |
025000 |
|
Using the Line field as a third key field defines the file so that
there are no duplicate keys.
| (First key field) order |
(Second key field) ordate |
(Third key field) line |
Item |
Qtyord |
Extens |
|---|
| 41834 |
062888 |
03 |
42111 |
30 |
020550 |
| 41834 |
062888 |
02 |
61132 |
04 |
021700 |
| 41834 |
062888 |
01 |
00623 |
50 |
025000 |
|
A logical file that has more than one record format can have records with
duplicate key values, even though the record formats are based on different
physical files. That is, even though the key values come from different record
formats, they are considered duplicate key values.