Lamda 表達式里的Join和GroupJoin的區別, 如何實現SQL的Left Join效果

例如，可以將產品表與產品類別表相聯接，得到產品名稱和與其相對應的類別名稱

db.Products  
.Join  
 (  
      db.Categories,  
      p => p.CategoryID,  
      c => c.CategoryID,  
      (p, c) => new {p,c.CategoryName}  
)  
.Where(p => p.CategoryID == 1);  

這個Join對應SQL里的inner Join, 這個很容易理解. 上面代碼里紅色的c的類型是Category

如果我要達到SQL里Left Join的效果, 該怎么寫呢? 想當然的以為,把Join改成GroupJoin,

 

db.Products  
.GroupJoin  
 (  
      db.Categories,  
      p => p.CategoryID,  
      c => c.CategoryID,  
      (p, cs) => new {p,cs.FirstOrDefault().CategoryName} 
)  
.Where(p => p.CategoryID == 1);  

 

上面代碼里紅色的cs的類型是IEnumerable<Category>, 你用FirstOrDefault()只是拿到一個目錄,萬一Product Left join是有多個目錄呢?

所以要用.SelectMany把IEnumerable<Category> 把里面的每一個元素做投影.

要注意加上DefaultIfEmtyp() 才是Left join, 假如沒有加這個,還是Inner Join

db.Products
.GroupJoin(db.Categories,
          (Product p) => p.CategoryId,
          (Category c) => c.CategoryId,
          (prod, cs) => new { prod, cs }) // cs is IEnumerable<Category>
          .SelectMany(prodCats => prodCats.cs.DefaultIfEmpty(), (prodCats, c) =>
                                new
                                {
                                    prodCats.prod
                                    categoryName = c.CategoryName
                                });

用LinQ query表達式 對應的寫法是

            var qry =
            from p in db.Products
            join c in db.Categories on  p.CategoryId equals c.CategoryId
            into g from pc in g.DefaultIfEmpty()
            select new { 
                    prodId = p.ProdId,
                    prodCode = p.ProdCode,
                    prodName = p.ProdName,
                    categoryName = g.FirstOrDefault().CategoryName
            };

 

如果我只用DefaultIfEmpty(),不用GroupJoin,這樣寫能達到這個效果嗎?

            var qry = db.Products.SelectMany
            (
                p => db.Categories.Where(c => c.CategoryId == p.CategoryId).DefaultIfEmpty(),
                (p, c) => new
                {
                    p.ProdId,
                    p.ProdCode,
                    p.ProdName,
                    c.CategoryName
                }
            );

這樣寫,如果數據量大,性能會很差. 因為它會把Products和Categories都Load到內存里,再Match, 而不是用SQL Left Join

 

參考文章: 

https://stackoverflow.com/questions/584820/how-do-you-perform-a-left-outer-join-using-linq-extension-methods

http://linqsamples.com/linq-to-objects/join/GroupJoin-linq

https://docs.microsoft.com/en-us/dotnet/csharp/linq/perform-left-outer-joins