Question

I want to make a MySQL to get daily differential values from a table who looks like this:

Date               | VALUE
--------------------------------
"2011-01-14 19:30" |   5
"2011-01-15 13:30" |   6
"2011-01-15 23:50" |   9
"2011-01-16 9:30"  |   10
"2011-01-16 18:30" |   15

I have made two subqueries. The first one is to get the last daily value, because I want to compute the difference values from this data:

SELECT r.Date, r.VALUE
    FROM table AS r
    JOIN (
    SELECT DISTINCT max(t.Date) AS Date
    FROM table AS t
    WHERE t.Date < CURDATE() 
    GROUP BY DATE(t.Date) 
    ) AS x USING (Date)

The second one is made to get the differential values from the result of the first one (I show it with “table” name):

SELECT Date, VALUE - IFNULL( 
    (SELECT MAX( VALUE ) 
    FROM table
    WHERE Date < t1.table) , 0) AS diff
    FROM table AS t1
    ORDER BY Date

At first, I tried to save the result of first query in a temporary table but it’s not possible to use temporary tables with the second query. If I use the first query inside the FROM of second one between () with an alias, the server complaints about table alias doesn’t exist. How can get a something like this:

 Date               | VALUE
 ---------------------------
 "2011-01-15 00:00" |   4
 "2011-01-16 00:00" |   6

Answer 1

Try this query –

SELECT
  t1.dt AS date,
  t1.value - t2.value AS value
FROM
  (SELECT DATE(date) dt, MAX(value) value FROM table GROUP BY dt) t1
JOIN
  (SELECT DATE(date) dt, MAX(value) value FROM table GROUP BY dt) t2
    ON t1.dt = t2.dt + INTERVAL 1 DAY