Aggregate functions (e.g. AVG, MIN, MAX, COUNT, etc.) can be in a SELECT statement without the GROUP BY clause. Without the GROUP BY clause in the SELECT statement will apply the aggregate function to the entire result set.
Also all aggregate functions, except for COUNT(*), ignore NULLs. So you need to be sure that when you use an aggregate function on a column, you want any NULL values in your column to be excluded in the aggregation, meaning they will not be included in the function.