Informix Error -2956
-2956 You may not set display field field-tag to a value because
none of its associated database columns belong to the same table as table, nor is it a variable display-only field.You cannot assign values in a LET statement across tables. All columns that are associated to the field-tag in a LET statement must belong to tables that are named in the opening command of the control block, or it must be a display-only field.