Thanks for answers, I just wonder where the operator precedence is documented, because from the online guide I see PowerBuilder gives "NOT" higher precedence to "AND" (from "operator precedence, in PowerBuilder expressions" page), and parenthesis suggested if I want override.

Have the AND higher precedence to NOT is pretty strange, 'tho. Never seen this on any language. But want confirm if that was the issue, will do some checks with your suggestions. Thanks again.

You should be using parenthesis around the "not isnull..." to assure the evaluation of the statements occurs in the order you expect.

Try these:

(not isnull(sku)) and (not isnull(location))

(not isnull(sku)) and isnull(location)  // these two should evaluate the same now
isnull(location) and (not isnull(sku))

isnull(sku) and (not isnull(location))

By not using the parens, you're evaluating "not (isnull(sku) and isnull(location))" instead of "(not isnull(sku)) and isnull(location)". It's all dependent on how the statement is evaluated. Different languages put precedence in different orders. Using the parens forces it to be consistent.

datawindow expressions have always used non-standard operator precedence 

so try using () to force the operator precedence that you want.

(not isnull(col1)) and isnull(col2)

vs

not (isnull(col1)) and isnull(col2))