We’ve already seen aggregate functions in action:
SELECT COUNT(*) FROM users WHERE country = 'us'
An aggregate function returns a value for the group of records. In our case, it gives us a number of records.
There’s another type of function in SQL – scalar functions. Unlike aggregate functions, they receive input (a column value, a user-defined value, like string/number or no input at all) and return a single value or a set of values. In the previous lesson, we saw a type-casting operator
::. In fact, it’s a shortcut of a
CAST scalar function: