Union / Minus operator in SQR

There are different scenarios where we will be using Union / Union all / Intersect / Minus operator in sqr.
below is the example of using union in sqr

begin-select 
field-a &fld-a 
field-b &fld-b 
from tbl1 
where ....
union 
select field-c, field-d from tbl2 where....
end-select