General SINGLE STATEMENT QUERY

I want to read the unit of measure from marc table.

The whole program was hard coded for one plant ,now they added few more materials under different plant.

My orginal statement was like this

select single ausme into itab-ausme

from marc

where matnr = itab-matnr

and werks = 'ABC'.

NOW IAM CHANGING IT TO.

select single ausme into itab-ausme

from marc

where matnr = itab-matnr

and werks = 'ABC'

OR WERKS = 'DEF'.

can anybody tell me how does it work

I thought it would first check plant ABC ,IF ITS NOT FOUND IN PLANT ABC THEN IT WOULD CHECK pLANT DEF.

lET ME KNOW WHETHER IAM RIGHT OR NOT?