Since you didn't specify, I will assume you mean in Oracle 10g. There are a lot of ways to see XML using SQL. By "see", I am also assuming that you mean create XML from relational data.
For selecting data using only SQL, there are the SQLX functions, XMLElement, XMLAGG, XMLattribute, etc. If you are using PL/SQL, in addition to SQLX, you can use DBMS_XMLQquery and to present XML on the Web you can use XSQL.
Without a more specific question, I think examples would be pointless. A great place to start would be Oracle's documentation Web site. The benfit of this site is well worth the time it takes for a free registration. You can search the documentation for XML, EMLElement, XMLType, XML DB,XDK and XMLQuery. Any of those will get you started. If you have a more specific question, please post it.
This was first published in June 2005