(?) SQL - interlinked joins (how to build a chain between two tables?)

First of all, that's not my table. I got a project with tables like that...

I need an idea to get all linked pages/questions in a queue/list in a given order.

Perfect soloution would be:

Do you know a way, to get this in one statement (via inner joins)? My problems are the order and also the mix between both tables, merged in one. It's also ok, if there are 2 id fields (one for each type), so than we also can go without type (for the current element).

You can use a couple of CTEs to first UNION the tables together and then follow through the list starting from the firstPage, using a row number to guarantee the ordering of results:

WITH allpq AS (
  SELECT name, 'page' AS type, next, next_type
  FROM page
  UNION ALL 
  SELECT name, 'question', next, next_type
  FROM question
),
list AS (
  SELECT type, name, next, next_type, 1 as rn
  FROM allpq
  WHERE name = 'firstPage'
  UNION ALL
  SELECT a.type, a.name, a.next, a.next_type, list.rn + 1
  FROM allpq a
  JOIN list ON a.name = list.next AND a.type = list.next_type
)
SELECT type, name, next, next_type
FROM list
ORDER BY rn

Output:

type        name            next            next_type
page        firstPage       secondPage      page
page        secondPage      firstQuestion   question
question    firstQuestion   secondQuestion  question
question    secondQuestion  thirdPage       page
page        thirdPage       fourthPage      page
page        fourthPage      fourthQuestion  question
question    fourthQuestion  fifthPage       page
page        fifthPage       fifthQuestion   question
question    fifthQuestion   sixthPage       page
page        sixthPage       seventhPage     page
page        seventhPage     eighthPage      page
page        eighthPage      

Demo on dbfiddle