我有以下查询:
SELECT 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')
我需要所有“emp_count”列的总和。目前,我遍历上述查询返回的所有行,然后在 Ruby 中对“emp_count”进行 SUM。但是有很多行,所以这会占用很多内存。
我如何在 SQL 中对“emp_count”求和并只返回这个数字?