sql选择并比较最近值和第n个最近值，但在值重复超过三个月时排除

我使用下面的查询从按MeterNumber分组的表中选择最近值和第三个最近值。用户提供一个月和一天来确定返回的最高值

查询的目的是查看一个水表的水读数，看看它们在3个月内是否相同。如果是这样，则不会发生消耗，并且仪表可能损坏

我的问题是，我还想排除任何具有相同读数的第4、5、6号仪表。。。最近读取的日期，因为这些仪表可能未使用

任何帮助都将不胜感激！我对这一点很陌生，所以也许有一种更有效/更好的方法来实现这一点

我正在使用MS SQL 2012

SELECT 
MeterNumber, 
MAX(WaterRead) AS CurrentRead, 
MAX(ReadDate) AS CurrentReadDate, 
MIN(WaterRead) AS nthLastRead, 
MIN(ReadDate) AS nthLastReadDate
From
(SELECT MeterNumber, ReadDate, WaterRead
    FROM (
        SELECT MeterNumber, ReadDate, WaterRead, Rank() over (Partition BY MeterNumber ORDER BY ReadDate DESC ) AS myRank
    FROM WaterReads
) WaterReads WHERE myRank <= 3 ) a
Group By MeterNumber
Having MAX(WaterRead) - MIN(WaterRead) = 0 AND MAX(WaterRead) != 0 AND MIN(WaterRead) != 0     AND MIN(ReadDate) <> MAX(ReadDate)
AND MONTH(MAX(ReadDate)) = 6 AND DAY(MAX(ReadDate)) = 25
ORDER BY MeterNumber, CurrentReadDate

---

你在正确的轨道上。您可以在以下情况下使用maxcase。。。将以前的特定时段提取出来进行比较。像这样：

SELECT
    meternumber,
    MAX(CASE WHEN myrank = 1 THEN readdate ELSE NULL END) curr_date,
    MAX(CASE WHEN myrank = 1 THEN waterread ELSE NULL END) curr_read,
    MAX(CASE WHEN myrank = 4 THEN readdate ELSE NULL END) ago3_date,
    MAX(CASE WHEN myrank = 4 THEN waterread ELSE NULL END) ago3_read,
    MAX(CASE WHEN myrank = 7 THEN readdate ELSE NULL END) ago6_date,
    MAX(CASE WHEN myrank = 7 THEN waterread ELSE NULL END) ago6_read
FROM
(        
    SELECT  MeterNumber, 
            ReadDate, 
            WaterRead, 
            Rank() over (Partition BY MeterNumber ORDER BY ReadDate DESC ) AS myRank
      FROM  WaterReads
            WHERE   readdate <= '20130625'
) AS a
GROUP BY
    meternumber
HAVING
    MAX(CASE WHEN myrank = 1 THEN waterread ELSE NULL END) = MAX(CASE WHEN myrank = 4 THEN waterread ELSE NULL END) 
    AND MAX(CASE WHEN myrank = 1 THEN waterread ELSE NULL END) > MAX(CASE WHEN myrank = 7 THEN waterread ELSE NULL END) 
    AND MAX(CASE WHEN myrank = 4 THEN waterread ELSE NULL END) > 0

或者，更容易阅读，但有一个附加的子查询：

SELECT
    meternumber,
    curr_date,
    curr_read,
    ago3_date,
    ago3_read,
    ago6_date,
    ago6_read
FROM    
(
    SELECT
        meternumber,
        MAX(CASE WHEN myrank = 1 THEN readdate ELSE NULL END) curr_date,
        MAX(CASE WHEN myrank = 1 THEN waterread ELSE NULL END) curr_read,
        MAX(CASE WHEN myrank = 4 THEN readdate ELSE NULL END) ago3_date,
        MAX(CASE WHEN myrank = 4 THEN waterread ELSE NULL END) ago3_read,
        MAX(CASE WHEN myrank = 7 THEN readdate ELSE NULL END) ago6_date,
        MAX(CASE WHEN myrank = 7 THEN waterread ELSE NULL END) ago6_read
    FROM
    (        
        SELECT  MeterNumber, 
                ReadDate, 
                WaterRead, 
                Rank() over (Partition BY MeterNumber ORDER BY ReadDate DESC ) AS myRank
          FROM  WaterReads
                WHERE   readdate <= '20130625'
    ) AS a
    GROUP BY
        meternumber
) AS b
WHERE
    curr_read = ago3_read
    AND curr_read > ago6_read
    AND ago3_read > 0

---

因此，如果当前读数等于上一次读取的第三次读数，您想显示结果，但如果当前读数等于上一次读取的第六次读数，则不想显示结果？@Joe-是的，这是正确的。谢谢！这是非常有用的，对我正在尝试做的事情很有用。我很感激你为我提供了一把小提琴和一个第二版本让我理解！

SELECT
    meternumber,
    curr_date,
    curr_read,
    ago3_date,
    ago3_read,
    ago6_date,
    ago6_read
FROM    
(
    SELECT
        meternumber,
        MAX(CASE WHEN myrank = 1 THEN readdate ELSE NULL END) curr_date,
        MAX(CASE WHEN myrank = 1 THEN waterread ELSE NULL END) curr_read,
        MAX(CASE WHEN myrank = 4 THEN readdate ELSE NULL END) ago3_date,
        MAX(CASE WHEN myrank = 4 THEN waterread ELSE NULL END) ago3_read,
        MAX(CASE WHEN myrank = 7 THEN readdate ELSE NULL END) ago6_date,
        MAX(CASE WHEN myrank = 7 THEN waterread ELSE NULL END) ago6_read
    FROM
    (        
        SELECT  MeterNumber, 
                ReadDate, 
                WaterRead, 
                Rank() over (Partition BY MeterNumber ORDER BY ReadDate DESC ) AS myRank
          FROM  WaterReads
                WHERE   readdate <= '20130625'
    ) AS a
    GROUP BY
        meternumber
) AS b
WHERE
    curr_read = ago3_read
    AND curr_read > ago6_read
    AND ago3_read > 0