Adding a constant IF not in an other table So I have this query.

SELECT * 
FROM `Comment` 
WHERE `Target` = '".$_GET['pp']."' 
AND `Id` NOT IN (SELECT `Target` 
                 FROM `Delete` 
                 WHERE `Type` = 'comment') 
ORDER BY `Id` DESC;


This selects comments which aren't deleted and displays them to members.

How could I modify it to select everything but with an additional boolean (called Deleted)?

I've heard about IF statements in SQL but don't know whether it's appropriate here or how it works.
Is this what you are looking for?  

    select * 
    FROM Comment cm left join [Delete] del
     on del.[Target]=cm.Id
    where cm.[Target] = '".$_GET[''pp'']."'
    and Type = 'comment'
    and del.[Target] is null
    and deleted=0