in.dat

原始数据——网络技术

abcdhgfe
123498765
You can create an index on any field, on several fields to be used
together, or on parts thereof, that you want to use as a key. The
keys in indexes allow you quick access to specific records and define
orders for sequential processing of a ISAM file. After you no longer
need an index, you can delete it. Addition and indexes have no effect
on the data records or on other indexes.
You may want a field in field in each record to uniquely identify that
record from all other records in the file. For example, the Employee
Number field is unique if you do not assign the same number to two
different employees, and you never reassign these numbers to other
employees. If you wish to find or modify the record belonging to a
specific employee, this unique field saves the thouble of determining.
If it is not the correct one, you must search again to find others.
If you know that you have a unique field within your records, you
can include this fact in the key description, and ISAM will allow only
unique keys. For example, if you specify that the employee numbers are
unique, ISAM only lets you add records to the file for, or change
numbers to, employee numbers that do not alreadly exist int file.