[英]LINQ-to-SQL: Select mulitple properties from top row of each GROUP using LINQ query
我有兩張表,Apps 和 AppGooglePlayMetadatas [AGPM],一個 Apps 行有很多 AGPM。 我想 select 一組應用程序,並且對於每個應用程序,還從每個最近的 AGPM 行返回一些屬性。 我可以用這段代碼做到這一點:
Apps
.Select(a => new
{
a.Name,
DatePublished = a.AppGooglePlayMetadatas.OrderByDescending(agpm => agpm.DatePublished).Select(agpm => agpm.DatePublished).First(),
CollectionDate = a.AppGooglePlayMetadatas.OrderByDescending(agpm => agpm.DatePublished).Select(agpm => agpm.CollectionDate).First(),
})
但這很丑陋,不能很好地擴展,並導致(可能)效率低下 SQL 訂購完成兩次:
SELECT [t0].[Name], (
SELECT TOP (1) [t1].[DatePublished]
FROM [AppGooglePlayMetadata] AS [t1]
WHERE [t1].[AppId] = [t0].[AppId]
ORDER BY [t1].[DatePublished] DESC
) AS [DatePublished], (
SELECT TOP (1) [t2].[CollectionDate]
FROM [AppGooglePlayMetadata] AS [t2]
WHERE [t2].[AppId] = [t0].[AppId]
ORDER BY [t2].[DatePublished] DESC
) AS [CollectionDate]
FROM [Apps] AS [t0]
這可以像這樣寫得更簡潔一些,但仍然會導致兩個 ORDER BY 語句:
Apps
.Select(a => new
{
App = a,
Latest = a.AppGooglePlayMetadatas.OrderByDescending(agpm => agpm.DatePublished).First()
})
.Select(a => new {
a.App.Name,
a.Latest.CollectionDate,
a.Latest.DatePublished,
})
我正在尋找一種方法來查詢連接表中特定行的多個屬性,從而生成高效的 SQL 查詢。 如果我要在 SQL 中寫這個,我相信我可以像這樣最有效地做到這一點:
SELECT TOP 100 a.Name, agpm.CollectionDate, agpm.DatePublished
FROM Apps AS a
CROSS APPLY (
SELECT TOP 1 DatePublished, CollectionDate
FROM AppGooglePlayMetadata AS agpm
WHERE a.AppId = agpm.AppId
ORDER BY DatePublished DESC
) AS agpm
不確定在這種情況下是否有可能讓 LINQ 生成 SQL 如此高效,但希望一些天才可以幫助我,謝謝!
您可以嘗試以下方法:
var result =
Apps.Join(
AppGooglePlayMetadata
.GroupBy(x => x.AppId)
.Select(grp => grp
.OrderByDescending(x => x.DatePublished)
.First()),
x => x.AppId,
x => x.AppId,
(app, meta) => new { app.AppId, meta.DatePublished, meta.CollectionDate });
這會讓你得到一個有效的 SQL 聲明嗎?
在實體框架中獲取“項目及其子項目”,如“學校及其學生”、“客戶及其訂單”、“應用程序及其 AGPM”,通常使用virtual ICollection<...>
那是一對多關系的一方面。 實體框架會將其轉換為正確的 GroupJoin。
如果您不想使用virtual ICollection<...>
,例如因為您認為可以更高效地執行此操作,請考慮自己執行 GroupJoin:
var result = Apps.GroupJoin( // GroupJoin Apps
AppGooglePlayMetadatas, // with AGPMs
app => app.Id, // from every App take the primary key
apgm => apgm.AppId, // from every Agpm take the foreign key to App
(app, AgpmsOfThisApp) => new // for every App with its zero or more AGPMs
{ // make one new
AppName = app.Name,
LatestAgpm = AgpmsOfThisApp.OrderByDescending(agpm => agpm.DatePublished)
.Select(agpm => new
{
CollectionDate = agpm.CollectionDate,
DatePublished = agpm.DatePublished,
}}
.FirstOrDefault(),
})
如果應用程序沒有 AGPM,則根本沒有 LatestAGPM,屬性 KatestAGPM 值將為 null
恕我直言,“帶有最新 AGPM 的應用程序”的概念清晰明了。 用法將是:
textBoxAppName.Text = result.AppName,
testBoxAgpmLastPublished.Text =
result.LatestAgpm.DatePublished;
如果真想混用App和AGPM的屬性,需要多加一個select:
.Select(appWithItsLatestAgpm => new
{
Name = appWithItsLatestAgpm.Name,
CollectionDate = appWithItsLatestAgpm?.CollectionDate,
DatePublished = appWithItsLatestAgpm?.DatePublished,
})
我不確定這是否會使結果的用戶更清楚。 DatePublished 是 App 的發布日期嗎?
用法將是:
textBoxAppName.Text = result.AppName,
testBoxAgpmLastPublished.Text = result.DatePublished;
這是否證明額外的 Select 是合理的?
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