Vælg den største værdi for hver sidste 7 dage

SELECT   DATE(FROM_UNIXTIME(time)) AS daygroup, 
         MAX(value) AS max_value, 
         MIN(value) AS min_value
FROM     tbl
WHERE    time >= UNIX_TIMESTAMP(CURDATE() - INTERVAL 7 DAY)
GROUP BY daygroup

Dette vil få minimum og maksimum af value for hver dag inden for de seneste 7 dage.

Rediger: Opdateret løsning til at hente data for hele rækken:

SELECT a.*
FROM tbl a
INNER JOIN
(
    SELECT   DATE(FROM_UNIXTIME(time)) AS daygroup, 
             MAX(value) AS max_value 
    FROM     tbl
    WHERE    time >= UNIX_TIMESTAMP(CURDATE() - INTERVAL 7 DAY)
    GROUP BY daygroup
) b ON DATE(FROM_UNIXTIME(a.time)) = b.daygroup AND a.value = b.max_value