An aggregate function can be used to perform calculations on a set of values, which will then return a single value. We can use an aggregate function either with the GROUP BY clause or without it.

SUM

SUM calculates the sum of given values.

Query to calculate the sum of all the salaries from the entire employee table:

`SELECT SUM(salary) as total_salary FROM employee;`

Query to calculate the sum of all the salaries for each department in the employee table:

`SELECT dept_id, SUM(salary) as total_salary_per_dept FROM employee GROUP BY dept_id;`

AVG

AVG calculates the average of given values.

Query to calculate the average of all the salaries from the entire employee table:

`SELECT AVG(salary) as avg_salary FROM employee;`

Query to calculate the average of all the salaries for each department in the employee table:

`SELECT dept_id, AVG(salary) as avg_salary_per_deptFROM employeeGROUP BY dept_id`

MIN

Find the minimum value from the given set of values.

Query to find the minimum salary from the entire employee table:

`SELECT MIN(salary) as min_salary FROM employee;`

Query to find the Minimum salaries for each department in the employee table:

`SELECT dept_id, MIN(salary) as min_salary_per_deptFROM employeeGROUP BY dept_id`

MAX

Find the maximum value from the given set of values.

Query to find the maximum salary from the entire employee table:

`SELECT MAX(salary) as max_salary FROM employee;`

Query to find the Maximum salaries for each department in the employee table:

`SELECT dept_id, MAX(salary) as max_salary_per_deptFROM employeeGROUP BY dept_id`

COUNT

COUNT returns the number of records.

Query to find the total number of records from the entire employee table:

`SELECT COUNT(emp_id) as no_of_emp FROM employee;`

Query to find the total number of employees in each department from the entire employee table:

`SELECT dept_id, COUNT(emp_id) as no_of_emp_per_dept FROM employee GROUP BY dept_id;`