Flex cannot *directly* connect to SQL Server or any other database for that matter.
You get data into flex using a HTTP layer: like for example using a ASP or ASP.NET page to generate XML -- or any other format of your choice JSON, CVS -- use HTTPService class, send a request to your server, get the data using result event of the class and do whatever you want to do wit this data.
Similarly, you'll need to post the data to the server over HTTP and store in the database.
There is no FDBC, like JDBC and ODBC!
No. You don't have to.
Consider a ASP.NET page. It can connect to SQL Server using ODBC driver -- or whatever MS call the driver these days -- you run your query and get a result set. you process the result set and convert it to an XML string. Maybe using some library or built-in support for XML in .NET framework. Now you XML ready. Just write to the Response object of your page.
Have you made any Ajax calls for data in your project? If so, you already know how to get this data in Flex. Using HTTPService class instead of XMLHTTPRequest.
Please checkout these two detailed blog entries (tutorials ) on Flex- SQL Server Connection.
Part1 - http://codeexpander.blogspot.com/2009/10/connecting-flex-to-sql-server.html
Part 2 - http://codeexpander.blogspot.com/2009/10/connecting-adobe-flex-to-sql-server.html
Hope this will help.