This document discusses SQL aggregate functions, group by, and having clauses. It defines aggregate functions as functions that return a single output value after aggregating multiple input values. Common aggregate functions listed are AVG(), COUNT(), MAX(), MIN(), and SUM(). Group by allows aggregation of columns by category and is used with select and aggregate functions. Having allows filtering of group by results after aggregation. Examples show using group by to aggregate average values by category and having to filter groups where the aggregate sum is over 1000.