Aggregate Function (Relational Operation)

A type of request that cannot be expressed in basic relational algebra is to specify mathematical aggregate functions on collections of values from the database.

We use the Aggregate Functional operation $\mathcal{F}$ for common aggregations:

• ${\mathcal{F}}_{\text{MAX Salary}}(\text{EMPLOYEE})$: retrieves maximum salary value from the relation
• ${\mathcal{F}}_{\text{MIN Salary}}(\text{EMPLOYEE})$: retrieves minimum salary value from the relation
• ${\mathcal{F}}_{\text{SUM Salary}}(\text{EMPLOYEE})$: retrieves the sum of the Salary from the relation
• ${\mathcal{F}}_{\text{COUNT Ssn, AVERAGE Salary}}(\text{EMPLOYEE})$: computes the count of employees and their average salary

We can also use a variation of the aggregate operation:

• Grouping attribute placed to the left.
• Aggregate functions placed to the right.

Hence, we can use ${}_{\text{DNO}}{\mathcal{F}}_{\text{COUNT Ssn, AVERAGE Salary}}(\text{EMPLOYEE})$ in order to group employees by their department number and compute the count of employees and average salary per department.