How to join tables without identifiers?
I finally figured that joining these tables would not bring a solution. I solved it doing this:

SELECT S.schaal, W.wnr, W.wnaam, W.salaris FROM Werknemer W, S_schaal S 
WHERE W.salaris &gt;= S.ondergrens &amp;&amp; W.salaris &lt;= S.bovengrens;