can you insert null value into date datatype in SQL (and if yes, how)? : I have been working on my SQL database for school project and i have a problem with my idea. I am doing a car service database (all made up) and i have a date data type for a collumn 'deadline' for a table 'order' (names for both in my language-'rok' and 'narocilo') and i am trying to insert null into some of rows where deadline isn't specified by the customer.  
  
<code>create table narocilo(
    id_narocila integer NOT null,
    opis varchar(50),
    cena integer,
    status varchar(20),
    datum_narocila date,
    rok date
);</code>  

this is some ways i tried to insert null but none of them worked  

<code>to_date('dd-mm-yyyy', null)
to_date(null)
null
to_date('null')</code>

So, my question is, can i insert null into date column and if yes, how?
Thank you in advance!