Explained the complete features of SQL Aggregate functions.
SQL count(*) in query is little tricky. This post explains why you need to use group by along with count aggregate function
SQL aggregate function and their rules. Built-in SQL functions.
SUM aggregate function explained how to use on multiple table columns and given sample SQL query.
Top differences of Aggregate functions and Scalar functions
The power of DB2 aggregate functions well explained with examples and syntax. Especially it covered all the rules related to SQL aggregate functions in a way dummies can understand quickly.