In addition to whole set of standard SQL aggregate functions, Virtuoso provides a method to create application-specific aggregate functions to create complex data structures inside SQL queries.

The most evident way to calculate some value that depend on number of rows of a table is to write a stored procedure that opens a cursor, then fetches row after row in a loop and repeatedly modifies some intermediate values ("accumulators") inside the loop. When the fetch operation signals that there are no more data, a final result is calculated from values of "accumulators"

E.g. to find an average of all values in a table's column COL, one may open a cursor to fetch values from the COL, then set two "accumulators" TOTAL and CNT to zero, then fetch row after row adding 1 to CNT and adding current value of COL to TOTAL. At the end of loop, an error should be signalled if CNT is zero, otherwise the result is TOTAL divided by CNT.

Obviously, a competent programmer will use built-in AVG() aggregate function inside a single SELECT statement: the code is much more readable and the use of aggregate reduces the overhead in filling in result-set and fetching from it.

More important advantage of aggregate functions is the ability to process many groups of records in parallel, performing one table scan instead of many. SQL optimizer may use sophisticated heuristics to find the fastest way of doing a complex query with aggregates but it cannot optimize the code of a stored procedure.