SQ02 - Query with adicional fields

Hi, I need your help to develop a query with additional fields.

The case is that:

In my query I only have a table VBAK and my propose is that this query returns me the quantity that are in a sales document for some material that should be selection parameters.

So I create 2 additional fields: one that is the u2018 Material u2018 that have no codification and other that is the quantity that have the code:

select single ZMENG into quantity from VBAK where matnr = material.

But the result is null, I don't know why.

Can you help me?

Thanks in advance.

Dora