Get configurable’s items quantity SUM using mysql

I was wondering if there is a way to get the sum of the stock of simple products in a configurable product using mysql.

As an example:

configurable sku = product_1

simple prodcuts associated to the configurable:

product_1_a qty = 10
prodcut_1_b qty = 5
prodcut_1_c qty = 5
prodcut_1_d qty = 0

I need to query the configurable sku “prodcut_1″and get the sum of the simple product (in this case 20)

Any suggestion?