mysql true way for using group by - mysql

Question

i have this table
CREATE TABLE IF NOT EXISTS `goldprice` (
`price` double unsigned NOT NULL,
`days` smallint(5) unsigned NOT NULL,
`seconds` mediumint(5) unsigned NOT NULL,
`sid` smallint(4) unsigned NOT NULL,
`gid` smallint(4) NOT NULL,
PRIMARY KEY (`days`,`seconds`,`sid`),
KEY `sid` (`sid`),
KEY `gid` (`gid`),
KEY `days` (`days`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
and i want minimum and maximum price of each day
first and last price of each day (base on its seconds)
with using subquery i can solve some part of my problem
before grouping i make a subquery and sorting in it
result in mysql is true
min, max, and last or first (based on sort type) can be made
two important things remains
last and first both is required
performance is very important subquery seems not good
and
i have his sql
SELECT price, days, seconds FROM goldprice
where gid=1 and days>=16200 group by days
order by days desc, seconds desc
change "days>=16200" to "days=16200"
will returns different result in "days=16200" row.
the sort is not remaining desc.
i know behavior of my sql group by
but i can't find good solution for my needs
MySQL order by before group by

Answer 1

Your query is incorrect. You select days with a random price match and a random seconds match. You should decide what price and seconds you want to show per day. The SUM? The MINinum? The MAXimum?
When starting with GROUP BY, you should make sure that for each column you either group by it or aggregate it (e.g. use SUM(price) instead of price alone or have price in the group by clause).
Example:
select a, b, MIN(c), MAX(d), e
from mytable
group by a, b;
a and b are okay, because you group by them. MIN(c) and MAX(d) are okay, because you aggregate c and d. e is incorrect; it is neither in the group by clause nor being aggregated. This is allowed in MySQL, but it's an advanced feature one must be aware of and handle that carefully. Above select statement would give just any of the matching e per a, b - the minimun e, the maximum e or just any other e. Only do this when you know that e is unique for a, b or you don't care what e you get. As said, it's an advanced feature.

Answer 2

my solve is here
select g2.days, group_concat(price) prices,
group_concat(seconds) times, minp, maxp from goldprice g1
inner join (
SELECT gid, days,
max(price) as maxp, min(price) as minp,
max(seconds) as maxt, min(seconds) as mint
FROM `goldprice` where gid = 1 and days = 16200
group by days
) g2
on (g1.days = g2.days and g1.gid = g2.gid and (seconds = mint or seconds = maxt))
where gid = 1 and days = 16200
group by days order by days desc, seconds asc
is my solve is correct