Aggregate functions
Aggregate functions operate on values across rows to perform mathematical calculations such as sum, average, counting, minimum/maximum values, standard deviation, and estimation, as well as some non-mathematical operations.
An aggregate function takes multiple rows (actually, zero, one, or more rows) as input and produces a single output. In contrast, scalar functions take one row as input and produce one row (one value) as output.
An aggregate function always returns exactly one row, even when the input contains zero rows. Typically, if the input contains zero rows, the output is NULL. However, an aggregate function could return 0, an empty string, or some other value when passed zero rows.