In order to illustrate how a migration will work, I have decided to migrate my (hypothetical) sports business from a custom solution based on Microsoft SQL Server to Salesforce.com.
Luckily Microsoft provide a good example of a legacy system : Adventureworks! You can download the database, attach it to whatever SQL Server instance you have (even the free Express edition) and treat that as your legacy source.
The great thing about the AdventureWorks is that it illustrates very well how differently two CRM systems can treat a customer. This is the data model of ‘Person’ in AdventureWorks, which we will have to migrate to Salesforce.com’s ‘Contact’ object:
We can already see that a good amount of the data structure is not required. We don’t have to migrate the data in ‘StateProvince’ and ‘CountryRegion’ because it already exists within Salesforce.com (although the values will need to be matched still). The ‘Password’ table should almost certainly not be migrated, although it leaves a tricky question about how to bring that (presumably) web site security login functionality to the new system. Many aspects are highly denormalized (limitless phone numbers, email addresses, etc).
Basically a simple mapping will not be possible. The flexibility of the relational database will make your migration task difficult. It is important to determine what data is truly important to you.