How to pickup null values in SQL Language?

I have written this function in SQL Language in postgresql 7.2.

CREATE FUNCTION MyFunction() RETURNS SETOF Text AS '

Select MyView."Document_ID"||','||MyView."First_Name"||','||MyView."Last_Name"||','||MyView."Sent_Date" "From MyView" Where "Document_ID"=$1;

' LANGUAGE 'SQL';

It worked fine...returning me multiple records of concatenated string,until there came a record where "Sent_Date" field was Null.It returned nothing then.

I want to put a check to set null values equal to zero(namely to set "sent_Date"=0 If it is Null) , but looks like SQL does not support "IF THEN ELSE".plz let me know how can i put that check in this function.?

Tahira Aslam