MySQL5: Delete all but the 50 newest rows

delete from table where id not in (
    select id from table
    order by id desc
    limit 50
)

SELECT unixTime FROM entries ORDER BY unixTime DESC LIMIT 49, 1;
DELETE FROM entries WHERE unixTime < $sqlResult;

SELECT timestampcol FROM table ORDER BY timestampcol DESC LIMIT 49,1;

DELETE FROM table WHERE timestampcol < ( SELECT timestampcol FROM table ORDER BY timestampcol DSEC LIMIT 49,1 )

DELETE FROM _table_
WHERE _date_ NOT IN (SELECT _date_ FROM _table_ ORDER BY _date_ DESC LIMIT 50)