calculate

* Single aggregate value: The single value is type cast to Fixnum for COUNT, Float
  for AVG, and the given column's type for everything else.

* Grouped values: This returns an ordered hash of the values and groups them. It
  takes either a column name, or the name of a belongs_to association.

    values = Person.group('last_name').maximum(:age)
    puts values["Drake"]
    # => 43

    drake  = Family.find_by(last_name: 'Drake')
    values = Person.group(:family).maximum(:age) # Person belongs_to :family
    puts values[drake]
    # => 43

    values.each do |family, max_age|
    ...
    end

Person.calculate(:count, :all) # The same as Person.count
Person.average(:age) # SELECT AVG(age) FROM people...

# Selects the minimum age for any family without any minors
Person.group(:last_name).having("min(age) > 17").minimum(:age)

Person.sum("2 * age")