select an entry with a NULL date field

Hello,

I have a problem with PostgreSQL when I try to select or delete an entry
with an empty date. That's a typical entry

Table tbl_date
-----------------------------
entry_id 154
date_01 2000-01-15
date_02 this date is NULL
name my_test
-----------------------------

I want to select every entry containing date_02 as NULL
I tried :
>select entry_id from tbl_date where date_02=NULL;
ERROR: parser: parse error at or near "null"
>select entry_id from tbl_date where date_02='';
Not work, that's a wrong date format
>select entry_id from tbl_date where date_02="";
Not work, "" considered as an attribute

Thank you for your time!
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Marc Andre Paquin