SQL query from hell - original from our source

queryFromHell.sql

SELECT
    p.sk_ID,
    IFNULL(
        IF(
            !ISNULL(c.sk_ID),
            c.sk_Verantwortlich,
            p.sk_Verantwortlich
        ),
        -1
    ) AS verantwortlich,
    DATE_FORMAT(
        p.sk_Time,
        IF(
            YEAR(NOW()) = YEAR(p.sk_Time),
            '%d.%m. %H:%i',
            '%d.%m.%Y %H:%i'
        )
    ) AS datum,
    IFNULL(
        p.sk_kt_ID,
        IF(
            LENGTH(TRIM(p.sk_WerWir)),
            p.sk_WerWir,
            '???'
        )
    ) AS sk_WerWir,
    p.sk_Kontakt,
    s_Schulnummer,
    s_ID,
    IF(
        ISNULL(s_NameErgaenzung),
        s_Name,
        CONCAT(s_Name, '<br>', s_NameErgaenzung)
    ) AS s_Name,
    sr_KurzName AS region_kurz
FROM
    s_Schulen AS s
JOIN
    sk_SchulKontakte AS p ON sk_Schule = s_ID
    # Nachtragkontakte sucht letzen Verantwortlichen raus
LEFT JOIN
    sk_SchulKontakte AS c ON c.sk_RefKontakt = p.sk_ID
LEFT JOIN
    sr_SchulRegionen AS sr ON sr.sr_ID = s.s_SchulRegion
WHERE
    p.sk_Erledigt = 'N' AND
    ISNULL(p.sk_RefKontakt) AND
    (
        ISNULL(c.sk_ID) OR
        c.sk_Time = (
            SELECT  MAX(c2.sk_Time) sk_time
            FROM sk_SchulKontakte c2
            WHERE c2.sk_RefKontakt = c.sk_RefKontakt
        )
    )
ORDER BY
    c.sk_Time DESC