How to create a SQL query without ActiveRecord?

I want to write an independent (not model-related) SQL query for some
statistic information
in an online shop system. So I know I could grab all users with
ActiveRecord and then just
count the array size, but that would massively waste resources. And I
also want MySQL to do
some max, min and average calculations. So is there a way to issue a
query and get the result set
without models being involved?

First of all, there are calculations available with ActiveRecord.

Person.count
  Person.count(:conditions => "age > 26")
  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](http://person.id)") # finds the number of rows matching the conditions and joins.

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

Or you can do Person.find_by_sql(“select first_name from people where age > 26”)

Retrieving the results is a little more interesting but not that bad. Search the API, there are lots of good things in there.

See http://api.rubyonrails.com/classes/ActiveRecord/Calculations/ClassMethods.html on calulations