vb.net - 如何使用日期记录在sql中唯一选择月份？

Question

我当前的系统有问题。我的数据库中有一个名为的表Payment。这是桌子的图片

如您所见，我对 3 月和 4 月的利息余额有相同的详细信息。但我希望resultset值是这样的：

我只想要currentIntBal每个月的一个细节。顺便说一句，我还需要知道如何将日期转换varchar为日期数据类型本身，因为该字段lastPaymentDate是varchar数据类型我怎么可能做到呢？

Answer 1

with monthlyPayments as
(
  select *
    , rownum = row_number() over (partition by year(lastPaymentDate), month(lastPaymentDate) order by lastPaymentDate)
  from payments
)
select lastPaymentDate
  , currentIntBal
from monthlyPayments
where rownum = 1

SQL Fiddle 与演示。

在这种情况下，我只是每月支付第一笔款项。

在附加信息后编辑：

with monthlyPayments as
(
  select *
    , rownum = row_number() over (partition by year(convertedDate), month(convertedDate) order by convertedDate)
  from payments p
    cross apply (select convertedDate = cast(replace(lastPaymentDate,'-','') as date)) c
)
select lastPaymentDate = convertedDate
  , currentIntBal
from monthlyPayments
where rownum = 1

在上面的新查询中，我将lastPaymentDate从更改varchar为date。我使用了 aCROSS APPLY所以它可以被多次引用而无需重复代码。

请注意，我还将日期格式更改为 ISO 格式，以防止不同连接语言出现任何问题 -YYYY-MM-DD可能会给出不一致的结果，但YYYYMMDD会保持一致。

SQL Fiddle 与演示。

Answer 2

像这样的东西：

SELECT MIN(lastPaymentDate) As lastPaymentDate, currentInBal
FROM   yourTable
GROUP BY currentInBal, MONTH(lastPaymentDate)+12*YEAR(lastPaymentDate)

另一种略有不同的方式：

SELECT MIN(lastPaymentDate) As lastPaymentDate, 
    MIN(currentInBal) As currentInBal
FROM   yourTable
GROUP BY MONTH(lastPaymentDate)+12*YEAR(lastPaymentDate)