You need to first load the field you want to check as a separate field and use the syntax below.

Table A:

Load

     UniqueField,

     UniqueField as UniqueTestField,

     Field1,

     Field3

From

SomeTable;

Table B:

Concatenate(Table A)

Load

     UniqueField

     Field1,

     Field4

From

OtherTable

WHERE not exists(UniqueTestField,UniqueField);