Question

我在SQL Server 2005中编写了一个正确的外连接查询，它工作正常，但我无法将其转换为LINQ。

这是我的问题：

select b.number, COUNT(*) AS [AudioCount] from audios a
right join months b on DATEPART(Month, a.[RecordedDate]) = b.number
group by number

请帮我转换为LINQ。

谢谢＆amp;问候， Anil Saklania

编辑：更正了查询。

Answer 1

根据您要查找的内容，我将其反转为左连接，但它是从数月到音频的左连接。这将使您在一个月没有录音时返回零计数。使用paolo的原始测试数据进行测试。

var audioMonths = from month in ListOfMonths
                    join audio in ListOfAudios on
                        month.number equals audio.RecordedDate.Month into audioLeftJoin
                    from audio in audioLeftJoin.DefaultIfEmpty()
                    select new
                    {
                        Month = month.number,
                        AudioId = audio != null ? audio.someProperty : null //Need some property on the audio object to see if it exists
                    };
var monthAudioCount = from audioMonth in audioMonths
                            group audioMonth by audioMonth.Month into grouping
                            select new
                            {
                                Month = grouping.Key,
                                AudioCount = grouping.Count(audioMonth => audioMonth.AudioId != null)
                            };

Answer 2

首先，本书的一些注释：J.＆amp ;;的LINQ Pocket Reference。 B. Albahari：
1.使用来自的额外转换为SelectMany 2. into 子句在 join 子句后直接出现时转换为GroupJoin。

Mike和Paolo的上述两个优秀解决方案都在查询中使用了第二个额外的 from 子句，因为它转换为SelectMany。使用SelectMany，“序列序列”（一系列音频序列）将转换为单个 flat 集合结果集。然后，为了对音频进行计数，在第二步中，单个平面输出集合根据月份进行分组。在上面的两个解决方案中，都已完成，并且它可以正常工作，但它还需要仔细检查空值。

开发自然层次。更简洁的替代方法是使用GroupJoin而不是SelectMany。 GroupJoin产生分层结果集，而不是SelectMany的平坦结果集。当然，分层结果集不需要分组，因此我们消除了第二步。

最重要的是，通过利用GroupJoin的分层结果集，我们不必检查空值。
因此，我们通过此代码实现另一个干净的左外连接，并借用Paolo的数据：

static void Main(string[] args) { var ListOfAudios = new List<Audio>() { new Audio() { someProperty = "test", RecordedDate = new DateTime(2011, 01, 01) }, new Audio() { someProperty = "test", RecordedDate = new DateTime(2011, 01, 02) }, new Audio() { someProperty = "test", RecordedDate = new DateTime(2011, 02, 01) }, new Audio() { someProperty = "test", RecordedDate = new DateTime(2011, 02, 02) } }; var ListOfMonths = new List<Month>() { new Month() {number=1, someMonthProperty="testMonth"}, new Month() {number=2, someMonthProperty="testMonth"}, new Month() {number=3, someMonthProperty="testMonth"} }; var q = from month in ListOfMonths join audio in ListOfAudios on month.number equals audio.RecordedDate.Month into hierarch select new { MonthNum = month.number, AudioCnt = hierarch.Count() }; foreach (var m in q) { Console.WriteLine("{0} - {1}", m.MonthNum,m.AudioCnt); } Console.ReadLine(); }

Answer 3

根据您的问题的一些评论，除了将您的查询翻译为linq之外，可能有更直接的方法来做您想做的事情。然而，就像练习一样，这是一种写作方式：

var res = from audio in ListOfAudios
    join month in ListOfMonths
    on audio.RecordedDate.Month equals month.number into joinAudioMonth
    from j in joinAudioMonth.DefaultIfEmpty()
    group j by j.number into g
    select new                             
    {
    number = g.Key,
    cnt = g.Count()
    };

修改上面的代码没有按照您的要求进行正确的加入，这是基于迈克回答的修改后的代码。这个不依赖于Audio对象的属性（即使对象本身存在，也可能为null）。但我很挑剔，迈克的回答基本上是正确的。

var audioMonths = from month in ListOfMonths join audio in ListOfAudios on month.number equals audio.RecordedDate.Month into monthAudioJoin from joined in monthAudioJoin.DefaultIfEmpty() select new { Month = month.number, J = joined }; var res = from audioMonth in audioMonths group audioMonth by audioMonth.Month into grouping select new { number = grouping.Key, cnt = grouping.Count(a => a.J != null) };

以下是我测试它的方式：

public class Audio { public string someProperty {get; set;} public DateTime RecordedDate {get; set; } } public class Month { public string someMonthProperty {get; set;} public int number {get; set; } } public static void Main (string[] args) { var ListOfAudios = new List<Audio>() { new Audio(){someProperty="test", RecordedDate=new DateTime(2011,01,01)}, new Audio(){someProperty="test", RecordedDate=new DateTime(2011,01,02)}, new Audio(){someProperty="test", RecordedDate=new DateTime(2011,02,01)}, new Audio(){someProperty="test", RecordedDate=new DateTime(2011,02,02)} }; var ListOfMonths = new List<Month>() { new Month() {number=1, someMonthProperty="testMonth"}, new Month() {number=2, someMonthProperty="testMonth"}, new Month() {number=3, someMonthProperty="testMonth"} // ... }; var audioMonths = from month in ListOfMonths join audio in ListOfAudios on month.number equals audio.RecordedDate.Month into monthAudioJoin from joined in monthAudioJoin.DefaultIfEmpty() select new { Month = month.number, J = joined }; var res = from audioMonth in audioMonths group audioMonth by audioMonth.Month into grouping select new { number = grouping.Key, cnt = grouping.Count(a => a.J != null) }; foreach(var r in res) { Console.WriteLine("{0} - {1}", r.number, r.cnt); }

LINQ右外连接问题

3 个答案: