PG UPDATE question

I'm new to postgres and am having an UPDATE problem when
using a join. I've got two tables like so:

CREATE TABLE QUESTION_ANSWERS (
QUESTIONS_ID int4 NOT NULL, -- question ID
ANSWERS_ID int4 NOT NULL, -- answer ID
CORRECT boolean
);

CREATE TABLE STUDENT_ANSWERS (
STUDENTS_ID int4 NOT NULL, -- student ID
QUESTIONS_ID int4 NOT NULL, -- question ID
ANSWERS_ID int4 NOT NULL, -- answer ID
CORRECT boolean
);

The table QUESTION_ANSWERS is completely populated. The table
STUDENT_ANSWERS has all fields except CORRECT populated. I want to
write an update that'll copy CORRECT from QUESTION_ANSWERS into
STUDENT_ANSWERS when the IDs match. Here's what I wrote:

UPDATE STUDENT_ANSWERS SET CORRECT=QA.CORRECT
FROM QUESTION_ANSWERS QA,STUDENT_ANSWERS SA
WHERE SA.CORRECT=null
AND SA.QUESTIONS_ID=QA.QUESTIONS_ID
AND SA.ANSWERS_ID=QA.ANSWERS_ID;

It executes without an error and correctly returns the count of the
records in the join, but the CORRECT field in STUDENT_ANSWERS is
populated entirely with false regardless of what CORRECT is
QUESTION_ANSWERS.

Can anyone shed light on what I'm missing?

--
Jeff Butera, Ph.D. Junk e-mail is like
Department of Mathematics & Computer Science plastic grocery bags:
High Point University they just seem to
jbutera@linus.highpoint.edu multiply at will...
http://linus.highpoint.edu/~jbutera Anonymous