seems I don't understand what you are trying to achieve. What's the benefit of loading the tables in that way instead of
if(field1 = 1, 'x',
if(field1 = 2, 'y',
if(field3 = 3, 'z'))) as created_field_2
Drop Table Master;
Rename table tmp to Master;
Or even doing the same in a preceding LOAD to your master table LOAD?