Question

Is it possible to change these 2 queries into 1?

Query #1:

UPDATE  `pvdownloader`.`posts`
SET  `exists` =  'y'
WHERE source = ANY (SELECT source FROM dbdownloader.posts)
  AND `mangacount` = 0
  AND NOT `exists` = 'm'

Query #2:

UPDATE `pvdownloader`.`posts`
SET  `exists` =  'n'
WHERE `exists` = 'u'
  AND `mangacount` = 0

I’m assuming it would be possible to make something like this with IF/ELSE but I can’t figure out how to use it at all :<

Answer 1

Maybe something like this would work:

update
  pvdownloader.posts
set
  exists =
    case
      when source = any(select source from dbdownloader.posts) and mangacount = 0 and exists != 'm' then 'y'
      when exists = 'u' and mangacount = 0 then 'n'
    end
where 
  (source = any(select source from dbdownloader.posts) and mangacount = 0 and exists != 'm')
  or
  (exists = 'u' and mangacount = 0)
;

As @MostyMostacho suggested, I took the liberty of changing your logic to not have a double-negative.