What is the SQL to delete a SQL User from a database?

Question

What is the SQL to delete a SQL User from a database?

Answer 1

 DROP USER [UserName]

You can prefix this with an existence check if required.

IF  EXISTS (SELECT * FROM sys.database_principals WHERE name = N'UserName')

There are some requirements to be met before dropping Users (e.g. they can't own any objects). Full Details here.