return value for date_format() when date field is NULL

Hi,

I cant find the answer on the manual pages. Maybe I should look harder.

Basically I'm putting dates into a table using:

then I can select dates that have put an INVOICEDATE in by doing

Only the reverse doesnt work, meaning I cant do a query for rows where no invoice date was entered (i.e. null)

So I need to know exactly what value the date_format function returns when it comes accross a null value. I've tried echoing the result to the screen and it returns an empty string.

This question was so intriguing I looked into my own database.
And indeed, if i looked for the records

select * from film where taal = null
I got not a single record, although I knew there had to be some.

Here's what I found at http://mysqld.active-venture.com/Pr..._with_NULL.html
If you want to search for column values that are NULL, you cannot use the =NULL test. The following statement returns no rows, because expr = NULL is FALSE, for any expression:

mysql> SELECT * FROM my_table WHERE phone = NULL;To look for NULL values, you must use the IS NULL test. The following shows how to find the NULL phone number and the empty phone number:

mysql> SELECT * FROM my_table WHERE phone IS NULL;mysql> SELECT * FROM my_table WHERE phone = "";And so I changed my SQL statement
Select * from film where taal = null
into
select * from film where taal is null

and this time, I got the records I expected.