linq中的左外連接

Question

我有以下查詢，但我不知道如何在表1上進行左外連接。

var query = (from r in table1
             join f in table2
                 on r.ID equals f.ID
             select new
             {     
                 r.ID, 
                 r.FirstName,
                 r.LastName,
                 FirstNameOnRecord = 
                     (f != null ? f.FirstName : string.Empty),
                 LastNameOnRecord = 
                     (f != null ? f.LastName : string.Empty),
                 NameChanged = 
                     (f != null 
                         ? (f.FirstName.CompareTo(r.FirstName) == 0 
                             && f.LastName.CompareTo(r.LastName) == 0) 
                         : false)
             }).ToList();

Answer 1

這是左外連接的一個很好的細分。

Answer 2

請參閱此示例或此示例以了解更多信息以及您的情況將會是這樣的 -

var query = from r in table1
            join f in table2
            on r.ID equals f.ID into g
            from f in g.DefaultIfEmpty()
             select new
             {     
                r.ID
                , r.FirstName
                , r.LastName
                , FirstNameOnRecord = (f != null ? f.FirstName : string.Empty)
                , LastNameOnRecord = (f != null ? f.LastName : string.Empty)
                , NameChanged = (f != null ? (f.FirstName.CompareTo(r.FirstName) == 0 
                &&  f.LastName.CompareTo(r.LastName) == 0) : false)
              }).ToList();

Answer 3

你見過這些例子嗎？ 你可能對這部分關於Linq的Left Outer Join感興趣。

Answer 4

使用lambda表達式

db.Categories    
  .GroupJoin(
     db.Products,
     Category => Category.CategoryId,
     Product => Product.CategoryId,
     (x, y) => new { Category = x, Products = y })
  .SelectMany(
     xy => xy.Products.DefaultIfEmpty(),
     (x, y) => new { Category = x.Category, Product = y })
  .Select(s => new
  {
     CategoryName = s.Category.Name,     
     ProductName = s.Product.Name   
  })