The big fat hairy-chested SQL server is usually the Data tier (the bottom) not the middle tier.
But the logical (design) and physical (implementation) layering may well be different.

You might end up with:
UI: VFP (VB or .NET)
Middle TierS (plural): VFP (my preference) or VB or .NET
Data tier: Oracle or SQL (perhaps VFP)

Some processing may be in the database server (packages or SPs) just for sheer speed and productivity.

AFAIK SQL server T-SQL has one big limitation, no way to scan a result set.
Oracle PL/SQL allows you to process records sequentially, and for our billing system that's a must!

For a web app it can get complicated:
IE might host the client side presentation layer code in VBScript or JavaScript.
IIS might host the "sort of" presentation layer - the interface objects to the business
IIS might also host the business layer
- or they could be on a remote machine (DCOM, SOAP etc)
IIS might also host the data access layer
- or they could be on a remote machine (DCOM, SOAP etc)
SQL Server or Oracle could host a further Data access layer (SPs, packages)
And finally you get to the 'real' data :-)


Implementation necessity is not the same as logical design might imply.