Microsoft SharePoint Server 2010 out-of-the-box comes with some new database-like features, e.g. unique field values and elements of referential integrity. Unfortunately native foreign keys are still not supported for list lookups. This gap makes it difficult to migrate or seamlessly integrate normalized external LOB data with foreign keys and referential integrity, e.g. from SQL databases, using integration tools like the
Business Data List Connector or others.
The Layer2 Key Value Lookup Column for SharePoint uses native keys (e.g. a Customer ID) for lookup values to another SharePoint list (e.g. customer name). To ensure integrity not the SharePoint Item ID is used (like with SharePoint out-of-the-box lookup)
but the foreign key itself. That solves many issues with SharePoint standard lookups, e.g. with deployment and updates.
Now it's possible: All your corporate master data, replicated to native SharePoint lists including foreign key relationships between... 
Fig.: Sample usage scenario of a Key Value Lookup Column in SharePoint 2010. You can look up the CompanyName in the Orders list from the Customers list using the CustomerID only as a key - not the SharePoint Item ID.