Here's another brand new feature in Component Designer 6 - a relational query editor. You would use this if you want to access an existing ("External" in Designer terms) RDBMS. First, you create a "Connection" which is a simple form that tells Designer what type of db it is and where it's located. Then, you can create a Relational Query that uses that connection.
Here's what the Relational Query editor looks like.
When you use the query in the page editor, you can see the XML representation in the Data Navigator in the lower right, in the picture below.
Link to download page.
Lotus Component Designer - Relational Queries