construct_calculation_sql

# File activerecord/lib/active_record/calculations.rb, line 152
        def construct_calculation_sql(operation, column_name, options) #:nodoc:
          operation = operation.to_s.downcase
          options = options.symbolize_keys

          scope           = scope(:find)
          merged_includes = merge_includes(scope ? scope[:include] : [], options[:include])
          aggregate_alias = column_alias_for(operation, column_name)
          column_name     = "#{connection.quote_table_name(table_name)}.#{column_name}" if column_names.include?(column_name.to_s)

          if operation == 'count'
            if merged_includes.any?
              options[:distinct] = true
              column_name = options[:select] || [connection.quote_table_name(table_name), primary_key] * '.'
            end

            if options[:distinct]
              use_workaround = !connection.supports_count_distinct?
            end
          end

          if options[:distinct] && column_name.to_s !~ /\s*DISTINCT\s+/i
            distinct = 'DISTINCT ' 
          end
          sql = "SELECT #{operation}(#{distinct}#{column_name}) AS #{aggregate_alias}"

          # A (slower) workaround if we're using a backend, like sqlite, that doesn't support COUNT DISTINCT.
          sql = "SELECT COUNT(*) AS #{aggregate_alias}" if use_workaround

          sql << ", #{options[:group_field]} AS #{options[:group_alias]}" if options[:group]
          sql << " FROM (SELECT #{distinct}#{column_name}" if use_workaround
          sql << " FROM #{connection.quote_table_name(table_name)} "
          if merged_includes.any?
            join_dependency = ActiveRecord::Associations::ClassMethods::JoinDependency.new(self, merged_includes, options[:joins])
            sql << join_dependency.join_associations.collect{|join| join.association_join }.join
          end
          add_joins!(sql, options, scope)
          add_conditions!(sql, options[:conditions], scope)
          add_limited_ids_condition!(sql, options, join_dependency) if join_dependency && !using_limitable_reflections?(join_dependency.reflections) && ((scope && scope[:limit]) || options[:limit])

          if options[:group]
            group_key = connection.adapter_name == 'FrontBase' ?  :group_alias : :group_field
            sql << " GROUP BY #{options[group_key]} "
          end

          if options[:group] && options[:having]
            # FrontBase requires identifiers in the HAVING clause and chokes on function calls
            if connection.adapter_name == 'FrontBase'
              options[:having].downcase!
              options[:having].gsub!(/#{operation}\s*\(\s*#{column_name}\s*\)/, aggregate_alias)
            end

            sql << " HAVING #{options[:having]} "
          end

          sql << " ORDER BY #{options[:order]} "       if options[:order]
          add_limit!(sql, options, scope)
          sql << ')' if use_workaround
          sql
        end