[sqlite] How are NULL values deleted?

[Tito Ciuro]

Hello,
I'd like to remove all rows with a specific column equaling NULL. I've 
tried this:

SQLite version 3.0.8
Enter ".help" for instructions
sqlite> select * from address;
Ciur�|1|Javi||Espa�a
Garaicoechevarria|2|Ana||Espa�a
Ciur�|3|Tito||Espa�a
Miti|4|Sam||Italy
Schmuck|5|Joe||Germany
sqlite> delete from address where ZIP = NULL;
sqlite> select * from address;
Ciur�|1|Javi||Espa�a
Garaicoechevarria|2|Ana||Espa�a
Ciur�|3|Tito||Espa�a
Miti|4|Sam||Italy
Schmuck|5|Joe||Germany
sqlite> delete from address where ZIP = 'NULL';
sqlite> select * from address;
Ciur�|1|Javi||Espa�a
Garaicoechevarria|2|Ana||Espa�a
Ciur�|3|Tito||Espa�a
Miti|4|Sam||Italy
Schmuck|5|Joe||Germany
Since all rows have NULL for ZIP, I would expect to remove all rows, 
but I cannot figure it out...

Any ideas?
Thanks,
-- Tito