I'm new to tableau, and am working on my first workbook.  I created an extract from a SQL Server datasource using custom SQL, published it to the server, created a workbook based on the extract, and published that on the server.  Now that I'm working on the workbook, I want to add one more field to the data extract.  How can I do this?  Is there a way to get back to the sql query and make this edit?  Can I pull the original query from the extract, change it and replace the old extract?