What is an aggregate function?
An aggregate function is a function such as count() or sum() that you can use to calculate totals. In writing expressions and in programming, you can use SQL aggregate functions to determine various statistics and values. Aggregate functions can greatly reduce the amount of coding that you need to do in order to get information from your database. (Excerpt from the PostgreSQL 7.1 manual) aggregate_name (expression) aggregate_name (ALL expression) aggregate_name (DISTINCT expression) aggregate_name ( * ) where aggregate_name is a previously defined aggregate, and expression is any expression that does not itself contain an aggregate expression. The first form of aggregate expression invokes the aggregate across all input rows for which the given expression yields a non-NULL value. (Actually, it is up to the aggregate function whether to ignore NULLs or not — but all the standard ones do.) The second form is the same as the first, since ALL is the default. The third form invokes the ag
