Which of the following is an example of an aggregate function?

An aggregate function is a type of function that performs a calculation on a set of values and returns a single value. The function is used in SQL queries to summarize data across multiple records. The SUM function specifically calculates the total of a numeric column, providing a single output value that represents the sum of all entries.

In the context of the other options, while SELECT is a way of retrieving data from a database, JOIN is a method for combining rows from two or more tables based on a related column, or WHERE is used to filter records based on specified conditions, none of these perform a calculation on a set of values or return a single summarizing value. Thus, SUM stands out as the only option that meets the criteria for an aggregate function, as it is used to compute the total amount across a selected data set.