mysql-SELECT在同一行中从同一表中获取两个条目,按日期区分

我有一张桌子,里面存放着不同的水表(水表,电表),而在另一张桌子里,我保留了每个水表的读数.
表结构是这样的：
仪表表

MeterID | MeterType | MeterName

读数表：

ReadingID | MeterID | Index | DateOfReading

每月读取仪表的读数.我现在想做的事情是在一排中获取仪表信息,当前读数和上一个读数.因此,如果我要查询,将导致以下行：

MeterID | MeterType | MeterName | CurrnetIndex | LastIndex

到目前为止,我有以下查询：

SELECT Meter.MeterID, Meter.MeterType, Meter.MeterName, CurrentReading.Index, PreviousReading.Index
FROM Meters AS Meter
LEFT OUTER JOIN Readings AS CurrentReading ON Meter.MeterID = CurrentReading.MeterID
LEFT OUTER JOIN Readings AS PreviousReading ON Meter.MeterID = PreviouseReading.MeterID
WHERE CurrentReading.ReadingID != PreviousReading.ReadingID AND DIMESTAMPDIFF(MONTH, CurrentReading.DateOfReading, PreviousReding.DateOfReading)=-1

问题是我可能没有当前读数或以前的读数,或两者都没有,但是我仍然需要检索仪表信息.获得NULL列对我来说是完全可以接受的,但是我仍然需要一行:)

解决方法:

采用：

   SELECT m.meterid,
          m.metertype,
          m.metername,
          current.index,
          previous.index
     FROM METER m
LEFT JOIN READING current ON current.meterid = m.meterid
                         AND MONTH(current.dateofreading) = MONTH(NOW())
LEFT JOIN READING previous ON previous.meterid = m.meterid
                          AND MONTH(current.dateofreading) = MONTH(NOW())-1

作为外部联接-如果在WHERE子句中完成MONTH过滤,则产生的结果与在ON子句中产生的结果不同.

标签：where-clause,sql,mysql,date
来源： https://codeday.me/bug/20191209/2095823.html