You will have to include the full logic expression in the where clause, like this:

.. where (expression from the column)='J'

Of course, in your case you could just do

.. where Detail.avr_wtg=.t.

>Dear readers,
>
>When i execute the folowing SQL query, i get the error colomn "WTG"not found.
>
>SELECT detail.avr_ari,;
> iif(Detail.avr_wtg=.t.,'J','N') as WTG;
>FROM general!detail;
>WHERE WTG='J'
>
>Is this an impossible kind of query, to include an expressionalias in the WHERE Clause ?
>
>Any idea's how to solve this ?
>
>- Bjorn