The mathematical functions that produce aggregated data are called aggregation functions. Aggregation functions **perform a calculation on a set of values and return a single value**.

The rules that apply to aggregate calculations are as follows:

- For any aggregate calculation, you cannot combine an aggregated value and a disaggregated value.
- Constant terms in an expression act as aggregated or disaggregated values as appropriate.
- All of the mathematical functions that produce aggregated data are called aggregation functions. Aggregation functions perform a calculation on a set of values and return a single value.