将两个MYSQL查询合并为一个查询

Question

I am trying to combine two MYSQL Queries into one. What I want to do is select the first and last row added for each day and subtract the last column for that day from the first column of that day and output that. What this would do is give me a net gain of XP in this game for that day.

Below are my two queries, their only difference is ordering the date by DESC vs ASC. the column in the database that i want to subtract from each other is "xp"

$query = mysql_query("

                SELECT * FROM (SELECT  * FROM skills WHERE
                userID='$checkID' AND
                        skill = '$skill' AND
                        date >= ".$date."
                ORDER BY date DESC) as temp
                GROUP BY from_unixtime(date, '%Y%m%d')


                ");

                $query2 = mysql_query("

                SELECT * FROM (SELECT  * FROM skills WHERE
                userID='$checkID' AND
                        skill = '$skill' AND
                        date >= ".$date."
                ORDER BY date DESC) as temp
                GROUP BY from_unixtime(date, '%Y%m%d')


                "); 

Answer 1

SELECT FROM_UNIXTIME(date, '%Y%m%d') AS YYYYMMDD, MAX(xp)-MIN(xp) AS xp_gain
FROM skills
WHERE userID = '$checkID'
AND skill = '$skill'
AND date >= $date
GROUP BY YYYYMMDD

This assumes that XP always increases, so it doesn't need to use the times to find the beginning and ending values.

If that's not a correct assumption, what you want is something like this:

SELECT first.YYYYMMDD, last.xp - first.xp
FROM (subquery1) AS first
JOIN (subquery2) AS last
ON first.YYYYMMDD = last.YYYYMMDD

Replace subquery1 with a query that returns the first row of each day, and subquery2 with a query that returns the last row of each day. The queries you posted in your question don't do this, but there are many SO questions you can find that explain how to get the highest or lowest row per group.