在ef下使用lambda实现left join

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     })