Consider a table object called Person. This table is having fields like age, first_name, last_name and salary.
Following will return the average age of all the employees.
The following will return the maximum age of the employees.
The following will return minimum age of the employees.
The following will return the sum of salaries of all the employees.
Person.sum :salary, :group => :last_name
The following will count the number of records having age more than 26.
Person.count(:conditions => "age > 26")
The following will count the total number of records.
Check the Calculation Functions link for more detail on Calculation Functions.