Hi Agnes & Gregory,

With some external help I was able to solve this problem. I used SQL Server (don't have VFP at home).

DECLARE @Schnaps TABLE 
    (   c1  CHAR(1), 
        c2  CHAR(1), 
        c3  CHAR(1) 
    )
    
    INSERT INTO @Schnaps VALUES ('A', 'a', '1')
    INSERT INTO @Schnaps VALUES ('A', 'b', '2')
    INSERT INTO @Schnaps VALUES ('A', 'b', '3')
    INSERT INTO @Schnaps VALUES ('B', 'c', '4')
    INSERT INTO @Schnaps VALUES ('B', 'c', '4')
    INSERT INTO @Schnaps VALUES ('B', 'd', '4')
    INSERT INTO @Schnaps VALUES ('B', 'e', '4') 
    INSERT INTO @Schnaps VALUES ('B', 'e', '4') 
    INSERT INTO @Schnaps VALUES ('B', 'f', '4') 
    INSERT INTO @Schnaps VALUES ('A', 'b', '3')
    /*
    SELECT T.* FROM @Schnaps T INNER JOIN 
    (SELECT c3, MIN(c1) AS c1_min,MAX(c1) AS c1_max, 
    MIN(c2) AS c2_min, MAX(c2) AS c2_max FROM @Schnaps X GROUP BY c3
    HAVING COUNT(DISTINCT(c1+c2)) > 1) X ON T.c3= X.c3
    */
    SELECT t1.*
   FROM @Schnaps t1 join(
    SELECT c1,c2,c3 FROM @Schnaps T
    GROUP BY c1,c2,c3 HAVING COUNT(*) =1) t2 ON t1.c3 =t2.c3 and t1.c2 =t2.c2
    and t1.c1 =t2.c1 WHERE t1.c3 in (SELECT c3 FROM @Schnaps X GROUP BY c3
    HAVING COUNT(DISTINCT(c1+c2)) > 1)