Question

IEnumerable<ViewEmployees> employees = from e in db.Users
                                       join p in db.Users on e.ParentID equals p.Id
                                       select new ViewEmployees
                                                  {
                                                       EmployeeName = e.Name,
                                                       EmployeeID = e.Id,
                                                       EmployeeCode = e.UserName,
                                                       ParentID = e.ParentID,
                                                       ParentName = p.Name
                                                  };

编写此查询是为了选择父级的名称并显示所有员工..

它只显示员工如何拥有父ID ..

我希望谁拥有parentId而谁不拥有......

Answer 1

生成两个集合的左外连接的第一步是使用组连接执行内连接。（有关此过程的说明，请参见如何：执行内部联接（C＃编程指南）。）在此示例中，Person对象列表基于与Pet.Owner匹配的Person对象内部连接到Pet对象列表。。第二步是将第一个（左）集合的每个元素包含在结果集中，即使该元素在右集合中没有匹配项也是如此。这是通过在组连接的每个匹配元素序列上调用DefaultIfEmpty来实现的。在此示例中，在匹配Pet对象的每个序列上调用DefaultIfEmpty。如果任何Person对象的匹配Pet对象的序列为空，则该方法返回包含单个默认值的集合，从而确保每个Person对象都在结果集合中表示。

public static void LeftOuterJoinExample()
    {
        Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" };
        Person terry = new Person { FirstName = "Terry", LastName = "Adams" };
        Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" };
        Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" };

        Pet barley = new Pet { Name = "Barley", Owner = terry };
        Pet boots = new Pet { Name = "Boots", Owner = terry };
        Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
        Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry };
        Pet daisy = new Pet { Name = "Daisy", Owner = magnus };

        // Create two lists.
        List<Person> people = new List<Person> { magnus, terry, charlotte, arlene };
        List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy };

        var query = from person in people
                    join pet in pets on person equals pet.Owner into gj
                    from subpet in gj.DefaultIfEmpty()
                    select new { person.FirstName, PetName = (subpet == null ? String.Empty : subpet.Name) };

        foreach (var v in query)
        {
            Console.WriteLine("{0,-15}{1}", v.FirstName + ":", v.PetName);
        }
    }

http://msdn.microsoft.com/en-us/library/bb397895.aspx

Answer 2

您可以像这样在临时变量User中获取父parent：

var employees = from e in db.Users
     let parent = db.Users.FirstOrDefault(p => e.ParentID == p.Id)
     select new ViewEmployees
     {
         EmployeeName = e.Name,
         EmployeeID = e.Id,
         EmployeeCode = e.UserName,
         ParentID = e.ParentID,
         ParentName = parent.Name
     };

您不需要检查parent != null，因为该语句已转换为SQL，不受空值的影响。

如果User具有导航属性Parent，则会更容易。

如何让所有可能拥有或不拥有ParentID的员工？

2 个答案: