LINQ外部连接有重复

Question

我在LINQPad中运行此查询。它的工作原理除了ProductSeries有重复的记录。

var query = from etaRecord in EtaRecord_0140 

    join productSeriesRecord in ProductSeries 
    on etaRecord.ProductSeriesID equals productSeriesRecord.ProductSeriesID 
    into productSeriesGroup 
    from productSeries in productSeriesGroup.DefaultIfEmpty() 

    where etaRecord.State == "A" 
    select new { EtaRecord = etaRecord, ProductSeriesRecord = productSeries }; 

query.Dump(); 

我尝试使用FirstOrDefault()，而不是DefaultIfEmpty()，但我得到这个错误：

An expression of type 'LINQPad.User.ProductSeries' is not allowed in a subsequent from clause in a query expression with source type 'System.Linq.IQueryable'. Type inference failed in the call to 'SelectMany'.

我怎样才能获得FirstOrDefault()为ProductSeries以便有只有一排每个EtaRecord？

.NET小提琴是在这里：https://dotnetfiddle.net/kRrold

Answer 1

你是说，你要加入的集合有重复？因为如果是这样，您可以提前将ProductSeries集合分组。

var query = from etaRecord in EtaRecord_0140 

join productSeriesRecord in ProductSeries.GroupBy(series => series.ProductSeriesID).Select(seriesGroup => seriesGroup.First()) 
on etaRecord.ProductSeriesID equals productSeriesRecord.ProductSeriesID 
into productSeriesGroup 
from productSeries in productSeriesGroup.DefaultIfEmpty() 

where etaRecord.State == "A" 
select new { EtaRecord = etaRecord, ProductSeriesRecord = etaRecord }; 

query.Dump(); 

现在，假设您正在使用静态列表，而不是数据库。如果它是一个数据库连接，那么你应该对结果做不同的处理。在事实之后可以以类似的方式完成。

Answer 2

它看起来像你需要分组：

var query = from etaRecord in EtaRecord_0140 

join productSeriesRecord in ProductSeries 
on etaRecord.ProductSeriesID equals productSeriesRecord.ProductSeriesID 
into productSeriesGroup 
from productSeries in productSeriesGroup.DefaultIfEmpty() 

where etaRecord.State == "A" 
group productSeries by new { etaRecord.ProductSeriesId, etaRecord } into g 
select new 
     { 
     EtaRecord = g.Key.etaRecord, 
     ProductSeriesRecord = g.Select(x => x).FirstOrDefault() 
     }; 

UPDATED FIDDLE

Answer 3

我会在一个子查询做到这一点：

var query = from etaRecord in EtaRecord_0140 
      where etaRecord.State == "A" 
      select new 
      { 
       EtaRecord = etaRecord, 
       ProductSeriesRecord = 
        (from productSeriesRecord in ProductSeries 
        where productSeriesRecord.ProductSeriesID == etaRecord.ProductSeriesID 
        select productSeriesRecord).FirstOrDefault() 
      }; 

在LINQ to对象，这可能是低效率的操作，因为为每个etaRecord执行子查询，但由于整个语句被翻译成SQL，因此查询优化器将处理优化的执行在计划。

这就是LINQ到实体的故事。

LINQ到SQL 总是似乎产生n + 1个查询，以组加入（其为join - into）与FirstOrDefault()组合。我已经尝试过几种场景，但是我无法让它只生成一个查询。我能找到生成一个查询的唯一的解决办法是：

var query = from etaRecord in EtaRecord_0140 
      where etaRecord.State == "A" 
      from productSeriesRecord in 
        ProductSeries 
         .Where(ps => ps.ProductSeriesID == etaRecord.ProductSeriesID) 
         .Take(1) 
         .DefaultIfEmpt() 
      select new { EtaRecord = etaRecord, ProductSeriesRecord = productSeries }; 

所以联接语法被放弃，以虚构的方式ProductSeries属于EtaRecord第一条记录进行查询。

Answer 4

的问题是你的额外from条款：

from productSeries in productSeriesGroup.DefaultIfEmpty() 

你应该抛弃这一点，只需使用：

let productSeries = productSeriesGroup.FirstOrDefault() 

...或者只是使用productSeriesGroup.FirstOrDefault()的select子句中，如下所示：

var query = from etaRecord in etaRecords    
      join productSeriesRecord in productSeriesRecords 
      on etaRecord.ProductSeriesId equals productSeriesRecord.ProductSeriesId 
      into productSeriesGroup 
      select new { EtaRecord = etaRecord, 
         ProductSeriesRecord = productSeriesGroup.FirstOrDefault() }; 

无论哪种更改，结果都是：

Snuh 1 - null 
Snuh 2 - null 
Snuh 3 - null 
Snuh 4 - Description A 
Snuh 5 - null 
Snuh 6 - Description B 

我想这就是你想要的。

Answer 5

您应该能够通过EtaRecord到额外的过滤步骤添加到组，只选择每个组的第一个记录

即

query = (from r in query 
     group r by r.EtaRecord.EtaId into results 
     select results.FirstOrDefault());