Complex Group-By Queries for XML C. Gokhale +, N. Gupta #, P. Kumar +, L.V.S. Lakshmanan *, R. Ng *, and B.A. Prakash + * University of British Columbia, Vancouver + Indian Institute of Technology, Bombay # Cornell University, Ithaca Why this paper? Of late, there is considerable interest in analytical processing of XML data. Even for data integration, there is a compelling need for performing various group-by style aggregate operations. A core operator needed for analytics is the group-by operator, which is widely used in relational as well as OLAP database applications. Why this paper? XQuery requires group-by operations to be simulated using nesting. State of current research: Provide support for grouping at the logical or physical level and recognize grouping operations from nested queries and rewrite them with grouping operations Extend XQuery FLWOR expressions with explicit constructs similar to the group-by, order-by and having clauses in SQL Direct algorithmic support for a group-by operator is not explored. Overview Nested Aggregation Expressing complex aggregation queries Having clause Moving Windows Related Work Algorithm MERGE-GB Extensions for disk-based Dealing with nesting Dealing with having clause Dealing with moving windows Experimental results Syntax Outer grouping attribute Inner grouping attribute Average of price Median of #Sold Syntax group //Book by //Name return ( //Name, avg(/Price), count(*) then by /Year return ( /Year, median(/#Sold) ) ) General Framework is an absolute XPath expression, while s and s are relative to . agg out s and agg in s are aggregation operations such as min(), count(), avg(), median(). Nested aggregations: medianMax(), minCount() Cons, AggCons out and AggCons in are sets of conditions. mw out s and mw in s denote moving window specifications. mw i (width, step, winType, domType) Nested Aggregation Features Having Clause agg i ( i ) i c i, where i in {=, , >,