From your question it is not clear whether you are trying to replace the desktop or to move MS SQL server to MySQL. There are a number of desktop tools that can provide a GUI SQL environment to replace your MS Windows application that interfaces to an SQL server.
I have not yet seen a connector for Linux desktops that will allow you to connect to an MS SQL server. There are tools like ksql, pgaccess and TkSQL that will allow you to connect to an MySQL or Postgresql backend server. These tools are available in open source both for MS Windows and for Linux/Unix. Additionally, the ODBC drivers for MySQL or Postgresql allow native MS applications like MS Excel and MS Access to access SQL backend data from MySQL and Postgresql respectively.
In most cases you will find the MySQL and Postgresql performance equal to or better than MS SQL running on tha same hardware under MS Windows 2000 Advanced Server.
>> For more information and links on ODBC, check out this definition on SearchVB.com.
This was first published in March 2003