How to calculate a value from two other table columns in my SQL

Can anyone help me understand how to create a result in a column from adding and subtracting values from two other tables/columns?

I have three tables each with a column named "qty":

qty_ad开发者_C百科d,  qty_pull,  qty_current

I need "qty_current" to reflect a result from the two other columns.

Erik

It is really hard to understand what exactly you want to do from your question but this might give you some direction:

SELECT part_no,
       SUM(qty) -
       ( SELECT SUM(qty) FROM cartons_pulled
         WHERE cartons_pulled.part_no = cartons_added.part_no ) AS current_qty
FROM cartons_added
GROUP BY part_no