java - 无法形成单个 MySql 查询来对表中的值求和

Question

"time"                  "id",      "parameter",  "value"
"2014-02-18 18:58:00",  "1$SGP",   "A",          "415.7806091308594"
"2014-02-18 18:58:00",  "1$SGP",   "B",          "240.3373565673828"
"2014-02-18 18:58:00",  "2$SGP",   "A",          "393.191162109375"
"2014-02-18 18:58:00",  "2$SGP",   "B",          "50.10090637207031"
"2014-02-18 18:58:00",  "3$SGP",   "E",          "3484841472"
"2014-02-18 18:05:01",  "1$SGP",   "A",          "0"
"2014-02-18 17:58:01",  "1$SGP",   "B",          "0"
"2014-02-18 17:58:01",  "2$SGP",   "C",          "0"
"2014-02-18 17:58:01",  "2$SGP",   "D",          "0"
"2014-02-18 17:58:01",  "2$SGP",   "E",          "3061691904"
"2014-02-18 17:57:01",  "3$SGP",   "A",          "0"
"2014-02-18 17:57:02",  "3$SGP",   "B",          "0"

现在，从具有上述数据的表 Foo 来看，我需要将所有不同 ID 的参数“A”的值相加，这样对于特定 ID，它会从之间的最后一个时间戳中获取值18:00:00 和 19:00:00。考虑到时间和 id 字段都已建立索引，如何编写一个快速满足此要求的单个查询。

Answer 1

认为您会很困难，因为您想要按时间提取，但您正在为日期/时间字段建立索引。

但是像这样的事情应该可以做到:-

SELECT SUM(value)
FROM foo
INNER JOIN
(
    SELECT id, MAX(`time`) AS maxtime
    FROM foo
    WHERE TIME(`time`) BETWEEN '18:00:00' AND '19:00:00'
    AND parameter = 'A'
    GROUP BY id
) sub1
ON foo.id = sub1.id
AND foo.`time` = sub1.maxtime