如何在JOIN擴展方法中實現左連接

Question

我試圖在p.Person表的這種查詢上實現外連接。 我該怎麼做？

此示例來自http://ashishware.com/DSLinqExample.shtml

var onlyinfo = p.Person 
    .Where(n => n.FirstName.Contains('a')) 
    .Join(p.PersonInfo, 
     n => n.PersonId, 
     m => m.PersonId, 
     (n, m) => m) 
    .ToArray<Persons.PersonInfoRow>(); 

Answer 1

通常左聯接在LINQ採取與組建模連接，有時與DefaultIfEmpty和SelectMany結合：

var leftJoin = p.Person.Where(n => n.FirstName.Contains("a")) 
         .GroupJoin(p.PersonInfo, 
            n => n.PersonId, 
            m => m.PersonId, 
            (n, ms) => new { n, ms = ms.DefaultIfEmpty() }) 
         .SelectMany(z => z.ms.Select(m => new { n = z.n, m)); 

這將使對一個序列（n，m）其中n是來自p.Person的條目和m是來自p.PersonInfo的條目，但是m如果沒有匹配，則爲空。

（這是完全未經測試，順便說一句 - 但應該給你的想法呢:)

Answer 2

對於LEFT OUTER JOIN試試下面的查詢。這是測試

var leftJoin = Table1 
       .GroupJoin(
           inner: Table2, 
        outerKeySelector: t1 => t1.Col1, 
        innerKeySelector: t2 => t2.Col2, 
         resultSelector: (t1, t2Rows) => new { t1, t2Rows.DefaultIfEmpty() } 
       ) 
       .SelectMany(z => 
        z.t2Rows.Select(t2 => 
         new { t1 = z.t1, t2 = t2 } 
        ) 
       ); 

Answer 3

如果有人遇到這個問題，並希望擴展方法來做到這一點，我用同樣的方法爲其他答案創建一個。它與常規聯接擴展方法具有相同的簽名。

public static IEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, 
     IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, 
     Func<TOuter, TInner, TResult> resultSelector) 
    { 
     return outer 
      .GroupJoin(inner, outerKeySelector, innerKeySelector, (outerObj, inners) => 
      new 
      { 
       outerObj, 
       inners= inners.DefaultIfEmpty() 
      }) 
     .SelectMany(a => a.inners.Select(innerObj => resultSelector(a.outerObj, innerObj))); 
    }