SELECT
CASE WHEN start = "0000-00-00" THEN NULL else start END as start,
CASE WHEN finish = "0000-00-00" THEN NULL else finish END as finish
FROM students

SELECT *
FROM openquery(OLDSYSTEM, '
    SELECT
    CASE WHEN start = "0000-00-00" THEN NULL else start END as start,
    CASE WHEN finish = "0000-00-00" THEN NULL else finish END as finish
FROM students')