PHP sql查询将两个dateTime字段减去毫秒（d-m-Y H：i：s.ms）

I have two fields in a sql query for a html table that needs to be subtracted. The datetime format looks like this "19/08/2017 23:32:08.580" My sql with inner joins looks like this:

$query = "SELECT tblentries.categories,
    tblracedetails.actual_start_time,
    tblentries.age,
    tblentries.firstname,
    tblentries.surname,
    tblentries.distance,
    tblentries.raceid,
    tblracetime.finishtime,
    tblracetime.raceno,
    tblentries.racedate,
    tblentries.racename,
    TIMEDIFF('tblracetime.finishtime','tblracedetails.actual_start_time') AS racetimes
FROM tblentries
INNER JOIN tblracetime
    ON tblentries.raceid = tblracetime.racesid AND tblentries.raceno = tblracetime.raceno
INNER JOIN tblracedetails
    ON tblracedetails.id = tblentries.racesid AND tblracedetails.distance = tblentries.distance
";

The data is stored in a varchar fields called "actual_start_time" and "finishtime". The final subtracted format should be h:m:s:ms and the subtracted value must be called "racetimes" . The sql query above displayed, works 100% except for an empty 00:00:00 racetimes column. Any suggestions will be appreciated? Thx

I got it working by changing that sql part to

TIMEDIFF(tblracetime.finishtime,tblracedetails.actual_start_time) times

and the actual field to a datetime with millisecond, using

ALTER TABLE tblracedetails ADD actual_start_time datetime(3)

and changing date format added to fields like so "yyyy-mm-dd h:m:s.ms" format