What is Aggregate Function?
An aggregate function summarises many rows into one value, such as COUNT, SUM, AVG, MIN or MAX.
Aggregate Function explained
Aggregates are the arithmetic of analysis: how many orders, total revenue, the average basket, the earliest date. Combined with GROUP BY they produce a summary per category.
Most aggregates ignore nulls, so AVG of a column with missing values averages only the rows that have one, which is a decision worth making deliberately.
Example
SELECT COUNT(*), SUM(amount), AVG(amount) FROM orders returns the number of orders, total revenue and average order value.