[sql] Select rows with same id but different value in another column

Join the same table back to itself. Use an inner join so that rows that don't match are discarded. In the joined set, there will be rows that have a matching ARIDNR in another row in the table with a different LIEFNR. Allow those ARIDNR to appear in the final set.

SELECT * FROM YourTable WHERE ARIDNR IN (
    SELECT a.ARIDNR FROM YourTable a
    JOIN YourTable b on b.ARIDNR = a.ARIDNR AND b.LIEFNR <> a.LIEFNR
)