Query of Queries (QoQ) is great little concept, for a while I never knew when I would ever use this up until a few months ago. Web services generally return strings, but more commonly xml. Then you have some minority that return query results. This is where QoQs work perfect, check out the code snippet below:
// Let's say qryResults has 4 columns - (Name, Address, Phone, Email)
SELECT Name, Address, Phone FROM qryResults
// Using QoQ you can modify this query to display only Name, Address, Phone
What are the benefits you may be asking?
- Perform table joins to different datasources
- Ability to access master detail information which you cannot get from the database details
- Manipulate cached query results for summary tables
- Need to use the query over and over in the same request? Greatly reduce access time, since the query is stored in memory (ideal for up to 50,000 records).
Give it a go, may be useful one day 
Tags: datasources, queries, tables