Having a unique ID for your data is important. When you first import your data into CiviCRM you can store this unique ID in the External Identifier field. When you subsequently update this data, you can use the external identifier as the field on which to match records. But sometimes you simply don't have a unique ID for contact data you want to import into CiviCRM.
Here we will use Google Refine to create our unique ID. Once we have this, we can use it to match contact data during CiviCRM import operations:
Download Google Refine from http://code.google.com/p/google-refine/wiki/Downloads?tm=2.
Import data from CSV, Excel, or a variety of other files into Google Refine to see how the import mechanism works. It may look something like this:
You can see that we cannot use Last Name values as a unique identifier as it is not unique. If we did an import using the last names, CiviCRM would only update the first matching record it found.
A combination...