How do I check for NULL

Hi, ALL,
Consider the following scenario:

CREATE TABLE test(a INT, b VARCHAR(256), c INT, d VARCHAR(256), /*
more fields follows*/);
CREATE UNIQUE INDEX test_x( b, c, d );

Now I try to do:

INSERT INTO test VALUES( 0, 'abc', 12345, (SELECT foo FROM bar),
/*more data follow*/);

My problem is:

The SELECT can either return data or NULL.
Everything is good when the data is returned, but the insert fails
when the NULL is returned, because the field "d" is a part of UNIQUE
INDEX.

However,, I'd like to still insert the record and I'd like to do something like:

INSERT INTO test VALUES( 0, 'abc', 12345, IF( (SELECT foo FROM bar) ==
NULL, "postgres", <select_result>), /*more data follow*/);

What would be the best way to achieve this?

Thank you..