Correct INSERT .. ON DUPLICATE KEY syntax?

Question

How can I check if a specific primary key (a string variable) already exists on the table and if not insert a new record otherwise just update the existing one with new values using c#?

I tried this

MySqlCommand cmd2 = new MySqlCommand("INSERT INTO mapdisplay    
    (ID,Distance) 
  VALUES 
    (@r,@c,) 
  ON DUPLICATE KEY UPDATE mapdisplay   
    (Distance) 
  VALUES 
    (@c,)", conn);

but I think the syntax is wrong.

Answer 1

INSERT INTO mapdisplay    
  (HexID,FlightNo,Lat,Lon,Alt,Course,Groundspeed,Verticalrate,Distance) 
VALUES (@r,@c,@f,@t,@w,@q,@u,@e,@y)  
ON DUPLICATE KEY UPDATE
  FlightNo = @c
  ,Lat = @f
  ,Lon = @t
  ,Alt = @w
  ,Course = @q
  ,Groundspeed = @u
  ,Verticalrate = @e
  ,Distance = @y;

Look at the last example in this link:
http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html

Answer 2

INSERT ... ON DUPLICATE KEY UPDATE ...