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Use cases:[2]
autoparts inventory (book keeping page statics of sales/sales person, parts, inventory on hand, etc)
internet statistics for through put on all interfaces on group of servers [0]
--- topic related books:
Joe Celko has book series 'SQL for Smarties' that covers basic through advanced aggrecation with sql using example queries.
Joe Celko's 'Trees and Hierarchies in SQL for Smarties' covers some of the ideas related to 'SQL statment returned a database'?
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[0]: https://dba.stackexchange.com/questions/173044/nested-aggreg...
[1] : http://unicon.org/
[2] : https://cs.uwaterloo.ca/~tozsu/courses/CS338/lectures/6%20Ne...