SUM() of COUNT() column in MySQL
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I have the following query:

    SELECT companies.*, employees.*, orders.*, COUNT(employees.id) AS emp_count 
    FROM `orders` 
    INNER JOIN `companies` ON `companies`.`id` = `orders`.`company_id` 
    INNER JOIN employees ON employees.company_id = companies.id
      AND (employees.deleted_at >= companies.activate_at OR employees.deleted_at IS NULL)                       
      AND employees.created_at <= companies.activate_at 
    WHERE 
      (companies.activate_at BETWEEN '2012-01-31 23:00:00' AND '2012-02-29 22:59:59'                    
       AND orders.type = 'Order::TrialActivation'
       AND orders.state = 'completed')

I need the SUM of all the "emp_count" columns. Currently I iterate over all the rows returned by above query, and then SUM on "emp_count" in Ruby. But there are many rows, so this takes up a lot of memory.

How would I SUM on "emp_count" in SQL and return just this number?