MySQL IF statement for ORDER BY clause issue - mysql

Question

This is a bit of a strange one: I have the following MySQL Stored Proc (or function) and the majority can be ignored safely unless there is a need to understand the full picture. The problem is the ORDER BY clause.
BEGIN
SELECT DISTINCT e.*, ( 3959 * acos( cos( radians(in_latitude) ) * cos( radians( e.address_latitude ) )
* cos( radians(e.address_longitude) - radians(in_longitude)) + sin(radians(in_latitude))
* sin( radians(e.address_latitude)))) AS distanceFromUsersPostcode
FROM event e
INNER JOIN event_organiser eo on e.event_organiser_id = eo.id
WHERE (e.event_name LIKE in_search OR e.address_town LIKE in_search OR e.address_county LIKE in_search OR eo.event_organiser_name LIKE in_search)
AND e.start_date_time >= in_start_date
AND e.start_date_time <= in_end_date
AND e.enabled = true
HAVING distanceFromUsersPostcode < in_maxDistanceFromUser
/*
* 1 *
ORDER BY distanceFromUsersPostcode
* 2 *
ORDER BY IF(in_orderBy='LOCATION', CAST(distanceFromUsersPostcode AS DECIMAL), e.start_date_time) ASC;
*/
ORDER BY
CASE in_orderBy
WHEN 'LOCATION' THEN distanceFromUsersPostcode
ELSE e.start_date_time
END
ASC;
END
Now the issue is this, the current uncommented ORDER BY clause seems to treat the DECIMAL value distanceFromUsersPostcode as a VARCHAR (or string) value.
It orders results in the form :
0.4, 101.9, 102.8, 11.1, 11.9
The same can be said if I used the variant labelled as * 2 *
However, if I revert to the original variant labelled as * 1 *, the results would be ordered as expected :
0.4, 11.1, 11.9, 101.9, 102.8
My guessing was that MySQL treated the distanceFromUsersPostcode variable as a VARCHAR if used inside the IF Function (* 2 *), hence my attempt to cast it to a DECIMAL. This however, has no effect.
Can anybody shed any light on what is happening here?
The following acts as expected but isn't very elegant of course as it duplicates the entire query :
BEGIN
IF in_orderBy='LOCATION' THEN
SELECT DISTINCT e.*, ( 3959 * acos( cos( radians(in_latitude) ) * cos( radians( e.address_latitude ) )
* cos( radians(e.address_longitude) - radians(in_longitude)) + sin(radians(in_latitude))
* sin( radians(e.address_latitude)))) AS distanceFromUsersPostcode
FROM event e
INNER JOIN event_organiser eo on e.event_organiser_id = eo.id
WHERE (e.event_name LIKE in_search OR e.address_town LIKE in_search OR e.address_county LIKE in_search OR eo.event_organiser_name LIKE in_search)
AND e.start_date_time >= in_start_date
AND e.start_date_time <= in_end_date
AND e.enabled = true
HAVING distanceFromUsersPostcode < in_maxDistanceFromUser
ORDER BY distanceFromUsersPostcode;
ELSE
SELECT DISTINCT e.*, ( 3959 * acos( cos( radians(in_latitude) ) * cos( radians( e.address_latitude ) )
* cos( radians(e.address_longitude) - radians(in_longitude)) + sin(radians(in_latitude))
* sin( radians(e.address_latitude)))) AS distanceFromUsersPostcode
FROM event e
INNER JOIN event_organiser eo on e.event_organiser_id = eo.id
WHERE (e.event_name LIKE in_search OR e.address_town LIKE in_search OR e.address_county LIKE in_search OR eo.event_organiser_name LIKE in_search)
AND e.start_date_time >= in_start_date
AND e.start_date_time <= in_end_date
AND e.enabled = true
HAVING distanceFromUsersPostcode < in_maxDistanceFromUser
ORDER BY e.start_date_time;
END IF;
END

Answer 1

Because you use different data types for sorting mysql will try to convert them to something convenient to be able to compare the values. In your particular case it will be conversion to string. So it makes no sense to convert distanceFromUsersPostcode to a number, because it will be converted back to string. You will need to transform the string in a convenient format for number sorting. LPAD function will help you here.
ORDER BY
CASE in_orderBy
WHEN 'LOCATION' THEN LPAD(CAST(distanceFromUsersPostcode as CHAR),6)
ELSE e.start_date_time
END
ASC;