Query designer in winforms app

You may find the Visual SQL Query Designer helpful. It will give you an idea that how you can implement this functionality.

This tool can be used to design SQL queries. Its UI is very basic as compared to SQL Server Management Studio. Its limitation is that it uses OLEDB connection string. You can download the source code from here.

EDIT:

EasyQuery.NET WinForms can be an option but its not free.

This article may also help you a bit.

One traditional "Microsoft" answer to this is to let them keep using Access... only point it at the SQL server and just let them build their custom queries there.

If you want to get fancy, you can build in a query user role and account, grant only read access to it, and even limit that baked-in role user account to some percentage of the total system load with the Resource Governor stuff if necessary.

There is no shame in prototyping new stuff in Access before rolling it up into real code, either.

Have you considered using the SSRS portion of SQL, and giving them access to the ReportBuilder tool? It has a decent interface for power users. If you have your primary keys and foreign keys set up in the database, it will recognize those relationships and assist the user in building multi-table queries. The tool can be downloaded directly from the SSRS web portal, you can integrate the reports directly into your .Net app using the ReportViewer control, or you can use simple HTTP requests to pull them back as Excel, PDF, etc.