Actually, you'd wanna select only the duplicate ones, since you don't want to deal with the ones that are fine already, right?

SELECT 
  DISTINCT * 
FROM 
  TABLE_A as A, 
  TABLE_B as B 
WHERE 
  A.ID = B.ID and 
  A.SECONDCOLUMN != B.SECONDCOLUMN
[download]

of course, doing a ..

SELECT
   COUNT(*)
FROM 
  (  
    SELECT ID, COUNT(*) AS CNT FROM TABLE_A GROUP BY ID
  ) as COUNTS
WHERE
  CNT = 1
[download]

will get you an idea of how many truely unique records there are. ++jeffa