insert/select with a condition

I'm not sure if this is possible but is there a way to select A unless its null then select B instead?

I am writing a trigger, my code is

insert into tbl_a(userid, obj)
select p.author, new.id 
FROM开发者_C百科 user_comment AS p 
WHERE p.id=new.parent

however new.parent is a nullable long (i'll switch to foreign key once supported in system.data.sqlite) if its null i get 0 results and no insert. I would like to use join media as m on m.id=new.media_id and return m.user_id if new.parent is null. So how do i write the select to return m.author if parent isnt null (which i'll return p.author like the above)

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You can use IFNULL(col1, col2). If col1 is not null, it is returned. If col1 is null, col2 is returned.