Question

以下是包含子查询的SQL查询。基本上我希望在给定的月份和年份获得我们设备的最新仪表读数。我知道这是在我的子查询中，问题出现了，但我不知道如何正确解决它。

如果我删除子查询并且只选择当月的仪表读数设备，则会产生以下结果。

eid eqid    name                    pid  hours  date
70  C1  KOMATSU WA250 3YD BUCKET    27  1176    2013-10-07
70  C1  KOMATSU WA250 3YD BUCKET    27  1195    2013-10-28
70  C1  KOMATSU WA250 3YD BUCKET    27  1178    2013-10-14
73  C11 KOMATSU PC200 EXCAVATOR     27  1080    2013-10-14
73  C11 KOMATSU PC200 EXCAVATOR     27  1099    2013-10-28
73  C11 KOMATSU PC200 EXCAVATOR     27  1078    2013-10-07
92  C4  CATERPILLAR 304D MINI EX    27  646     2013-10-14
92  C4  CATERPILLAR 304D MINI EX    27  645     2013-10-07
92  C4  CATERPILLAR 304D MINI EX    27  649     2013-10-28
58  E14 BOBCAT-ATV 2300 Utility     8   522     2013-10-31
61  E17 SKYTRAK FORKLIFT            40  943     2013-10-10
62  E18 SKYTRAK FORKLIFT            5   1790    2013-10-30
62  E18 SKYTRAK FORKLIFT            5   1789    2013-10-29
62  E18 SKYTRAK FORKLIFT            5   1777    2013-10-13
62  E18 SKYTRAK FORKLIFT            5   1772    2013-10-07
62  E18 SKYTRAK FORKLIFT            5   1777    2013-10-13
62  E18 SKYTRAK FORKLIFT            5   1772    2013-10-04
62  E18 SKYTRAK FORKLIFT            5   1772    2013-10-07
62  E18 SKYTRAK FORKLIFT            5   1772    2013-10-04
67  E23 BOBCAT SKID STEER           27  1178    2013-10-28

以下是完整查询的结果，包括子查询。

92  C4  CATERPILLAR 304D MINI EX    27  649    2013-10-28
61  E17 SKYTRAK FORKLIFT            40  943    2013-10-10
62  E18 SKYTRAK FORKLIFT            5   1790   2013-10-30

以下是我正在使用的查询。

SELECT e.eid, e.eqid, e.name, m.pid, m.hours, m.date FROM meter m
JOIN equipment e ON m.eid = e.eid
WHERE MONTH(date) = $month
AND YEAR(date) = $year
AND m.date = (SELECT MAX(m2.date) FROM meter m2 WHERE m2.eid = m.eid)
ORDER BY e.eqid ASC

非常感谢任何帮助。

EDIT *** 我本来就没有塞巴斯。我不得不改变一件事而且效果很好。

SELECT DISTINCT e.eid, e.eqid, e.name, m.pid, m.hours, m.date 
FROM equipment e
JOIN (
    SELECT eid, MAX(date) as date
    FROM meter
    WHERE MONTH(date) = $month
    AND YEAR(date) = $year
    GROUP BY eid
) maxdate ON maxdate.eid = e.eid
JOIN meter m ON m.eid = e.eid AND m.date = maxdate.date
ORDER BY e.eqid ASC

Answer 1

猜出每个表的字段，我想出了以下查询：

SELECT DISTINCT e.eid, e.eqid, e.name, m.pid, m.hours, m.date 
FROM 
    equipment e
    JOIN (
        SELECT eid, MAX(date)  date
        FROM meter
        WHERE 
            MONTH(date) = $month
        AND YEAR(date) = $year
        GROUP BY eid
    ) maxdate ON maxdate.eid = e.eid
    JOIN meter m ON m.eid = e.eid AND m.date = maxdate.date
ORDER BY e.eqid ASC

Answer 2

尝试

SELECT e.eid, e.eqid, e.name, m.pid, m.hours, m.date
  FROM 
(
  SELECT eid, MAX(date) date
    FROM meter
   WHERE date BETWEEN '2013-10-01' AND LAST_DAY('2013-10-01')
   GROUP BY eid
) q JOIN meter m 
    ON q.eid = m.eid 
   AND q.date = m.date JOIN equipment e
    ON q.eid = e.eid
 ORDER BY e.eid

注意：确保您在date上有索引。不要将任何函数（MONTH()等）应用于WHERE子句中的这些列，因为它会阻止您使用任何可能导致全部表扫描的索引。

示例输出：

| EID | EQID |                     NAME | PID | HOURS |       DATE |
|-----|------|--------------------------|-----|-------|------------|
|  58 |  E14 |  BOBCAT-ATV 2300 Utility |   8 |   522 | 2013-10-31 |
|  61 |  E17 |         SKYTRAK FORKLIFT |  40 |   943 | 2013-10-10 |
|  62 |  E18 |         SKYTRAK FORKLIFT |   5 |  1790 | 2013-10-30 |
|  67 |  E23 |        BOBCAT SKID STEER |  27 |  1178 | 2013-10-28 |
|  70 |   C1 | KOMATSU WA250 3YD BUCKET |  27 |  1195 | 2013-10-28 |
|  73 |  C11 |  KOMATSU PC200 EXCAVATOR |  27 |  1099 | 2013-10-28 |
|  92 |   C4 | CATERPILLAR 304D MINI EX |  27 |   649 | 2013-10-28 |

这是 SQLFiddle 演示

date子查询PHP和Mysql

2 个答案: