[Rails.example] count

ref

Person.count("age > 26")

Person.count('id', :conditions => "age > 26")
# Performs a COUNT(id)

Person.count(:all, :conditions => "age > 26")
# Performs a COUNT(*) (:all is an alias for '*')

Person.count(:conditions => "age > 26 AND job.salary > 60000", :include => :job)
# because of the named association, it finds the DISTINCT count using LEFT OUTER JOIN.

Person.count(:conditions => "age > 26 AND job.salary > 60000", :joins => "LEFT JOIN jobs on jobs.person_id = person.id")
# finds the number of rows matching the conditions and joins.