title: which values belong to two persons
body: <p>I've got these tables:</p>
  
  <pre><code>Tabel Instrument
  Instrument  Pitch
  drums       null
  guitar      null
  paino       null
  
  Tabel Occupation
  Stuknr Instrument
  1      guitar
  2      piano
  2      guitar
  3      piano
  4      drums
  4      guitar
  
  Tabel Music
  Stuknr ComposerID
  1      1
  2      1
  3      2
  4      2
  
  Tabel Composer
  ComposerID ComposerName
  1          Test1
  2          Test2
  </code></pre>
  
  <p>Now i need a query thats:
  Which instrument are founds in both music of the composers</p>
  
  <p>my attempt:</p>
  
  <pre><code>SELECT I.instrumentnaam
  FROM Instrument I 
  WHERE EXISTS (SELECT 1
            FROM Bezettingsregel B
            WHERE B.instrumentnaam = I.instrumentnaam
            AND EXISTS(SELECT 1
                       FROM Stuk S
                       WHERE S.stuknr = B.stuknr
                       AND EXISTS(SELECT 1
                                  FROM Componist C
                                  WHERE C.componistId = S.componistId AND C.naam = 'Karl Schumann' OR  C.naam = 'Thom Guidi')))
  </code></pre>
  
  <p>But this show all instruments for both composer, i want only the instruments that both composer use.</p>
  
  <p>Thanx for help!</p>