Sometimes I need to make queries and join tables, knowing that the
other table rows are not always available.

Example:

I do

SELECT question.author, ..., textbooks.picture
WHERE textbooks.id = questions.tb_id

the problem is that sometimes questions.tb_id is NULL (textbook was
not supplied in question), and what I want to have for
textbook.picture is a NULL. But I want the row returned if it exists
in questions.

How would I do it?

o