How to (MySQL) multiply columns and then the sum rows?

Question

I've got this table:

id    |   payment_id   | quantity  |  unit cost
1           633             1            750  
2           633             1            750 
3           632             2            750 
4           632             2            750 

What I need is :

payemnt_id  |   total
    633           1500
    632           3000 

You can also think of this as Countries and then you are trying to find the total for each Country. I was sure there were some tutorials like this but could not find by STFW.

Answer 1

You can simply put the formula (expression) inside SUM:

SELECT payment_id, SUM(`unit cost` * quantity) AS total
FROM myTable
GROUP BY payment_id