Just a question ... For example, I want to develop a payroll system for a business. Is it legal to use SQL Server Express Edition?
Yes. YES. YES!!! Totally, absolutely FREE for anyone who can use it in development and production.
On the appropriate Microsoft website :
Easy to get to. Free to use .Available for free for development and deployment .Quickly create and deploy a solution for your database.Backup and restore with ease.Available with graphical management tool, reporting capabilities, full-text search and spatial support.Ideal for building websites and applications, small LOB business solutions, or for redistribution with ISV applications.
Easy to get to. Free to use .
As shown on MSDN , you can use SQL Server Express Edition in a production environment, but you must consider some technical limitations: