SQL COUNT bei neuer Gruppe neu beginnen

[woki]

Oder auch:

SELECT (Select count(1)                
        From MyTable as b     
        Where b.NR = a.NR      
          and b.ANZ < a.ANZ + 1),
       a.NR, a.ANZ, a.WERT              
FROM MyTable as a             
ORDER BY a.NR, a.ANZ