title: Find the sids of suppliers who supply only red parts
body: <p>again a sql question.</p>
  
  <p>First thing i do is to join the Tables:
  Select SID from cat c JOIN part p ON c.PID=p.PID;
  I tried the having clause but i have no aggregate function and using a 'where' is useless because then i get all suppliers which supply a red part.</p>
  
  <p>Is there a generalized answer ? so that i can get also the supplier which provides only green parts ?</p>
  
  <p>The output needed:</p>
  
  <pre><code>SID   
  -----   
  S2
  </code></pre>
  
  <p>The Tables provided:</p>
  
  <pre><code>SELECT * FROM parts 
  
  PID   PNAME      COLOR      WEIGHT CITY       
  ----- ---------- ---------- ------ ---------- 
  P1    Nut        red            13 London     
  P2    Bolt       black          18 Paris      
  P3    Screw      red            17 Rome       
  P4    Screw      silver         14 London     
  P5    Cam        trans          12 Paris      
  P6    Cog        cyan           19 London     
  P7    Nut        magenta        15 -          
  P8    Wheel      red            15 Munich     
  P9    Bearing    green          15 Milano     
  
    9 record(s) selected. 
  
  
  SELECT * FROM catalog 
  
  SID   PID   COST         
  ----- ----- ------------ 
  S1    P3            0.50 
  S1    P4            0.50 
  S1    P8           11.70 
  S2    P1           16.50 
  S2    P3            0.55 
  S2    P8            7.95 
  S3    P8           12.50 
  S3    P9            1.00 
  S4    P5            2.20 
  S4    P6      1247548.23 
  S4    P7      1247548.23 
  
    11 record(s) selected.
  </code></pre>
  
  <p>Thank you for your great help.</p>