This can be done through a concept called Data Blending.Please find the below links in regards to the same.
Let me know if yopu require any further information.
Thanks for the reply and the informative links.
Does this mean that I can design a report/dashboard of 2 different databases for example: SQL Server and Oracle; so that the user has an option to select either Oracle or SQL data source as a prompt at runtime to get the live data.
Not exactly.The concept of data blending means you can combine the data of two different databases by using a join(Common Object).
For example: I have a SQL Server and an Oracle environment.Now using datablending I can bring the combined data of the two datasources using a common object(It should be present in both datasets) between the two databases.
Well, you kinda can if you specifically choose not to have a blend. All data shown would have to be aggregated in some way. See attached, but there's every chance it won't match your needs.
Book2.twbx.zip 1.2 MB
an option to select either Oracle or SQL data source as a prompt at runtime to get the live data
I don't think you can replace the data source dynamically for the same visualization, but you can run the same parameter against multiple visualizations in the same dashboard at the same time, or design a parameter for Oracle and another for SQL Server if you want to run them separately.
There are also various techniques to show and hide views in a dashboard and thus do what you want, hmm ... actually I don't know what you want, but guess you want to know what is possible.
Here is an example that runs the same parameter live simultaneously against Oracle and SQL Server:
Data source: Oracle
WHERE NAME LIKE '%'||replace(<Parameters.Name Parameter>,' ','%')||'%'
Data source: SQL Server
WHERE NAME LIKE '%'+replace(<Parameters.Name Parameter>,' ','%')+'%'
Obs. Custom SQL Parameters is a new feature in Tableau 8
Thanks Johan, Alex for a descriptive and helpful answer. This answers my question.