|
|
When information is sorted or merged, keys determine the output
record sequence. Keys are defined by their beginning position, length,
key type, and ordering sequence (ascending or descending).
For example, to sort by last names with the record format
below, you would specify a key that begins in column 1 and is 20
characters long, byte (ASCII) type, and ascending sequence.
Last Name First Name Employee Number Hire Date
Jackson, Jonathan 000006 06/06/87
0 1 2 3 4 5 6
123456789012345678901234567890123456789012345678901234567890123456789
You can specify the collating sequence, which is the order
by which the keys are sorted. You can use a predefined order, like
ASCII, EBCDIC, or a Native Language, or you can define your own
collating sequence.
You may use multiple keys. The major key determines the part of the record
examined first in the sort. As Figure 1-4 "Key
Comparing Operations" shows
if the major keys of two records are the same, the secondary keys
determine the new sequence of the records. If two records have the
same first keys and the same second keys, their third keys are compared
to determine the sequence. If all the key fields in two or more
records are identical, the output file preserves the order of the
input records.
Figure 1-4 Key Comparing Operations
For more information about specifying keys, refer to Chapter 2.
For more information about collating sequences, refer to Chapter 4.
|