Fabio,

>and these conditions have to return NULL (unknow),
>a condition that skip the row.

So far as a know to skip the row the select clause need to return .F.

Foxpro is not able to handle a unknow (.NULL.) where clause
that's why we have the ISNULL() function.

This code give a invalid where clause

CLEAR
CREATE CURSOR _TABLE (F1 INT NULL)

SELECT 2 FROM _TABLE T1 WHERE .NULL. INTO ARRAY zz

Marcel