Question

我需要Linq自我加入的一些帮助。

我有以下课程：

public class Owner
{
public string OwnerId {get;set;}
public string Name {get;set;}
public string Area {get;set;}
public string City {get;set;}
public string Sex {get;set;}
public List<Dog> dog {get;set;}
}

并且表......

ID OwnerId OwnerName TypeId TypeName   TypeValue        TypeCodeId
1     1     John       1     Area      United States       440
2     1     John       2     City      Los-Angeles         221
3     1     John       3     Sex       Female              122
4     2     Mary       1     Area      Mexico              321
4     2     Mary       2     City      Cancun              345
............................................................

我需要以最快的方式将table1的结果解析为所有者列表。注意：Type可以为null，但我仍然需要显示该所有者（所以，我假设左连接应该有效）。

这是我的所作所为。（所有者是包含table1结果的Web服务类）

public IEnumerable<Owner> GetOwners() { 
  return  (from owner in owners 
          join area in owners into owner_area
          from oa in owner_area.DefaultIfEmpty()
          join City in owners into owner_city
          from oc in owner_city.DefaultIfEmpty()
          join sex  in owners into owner_sex
          from os in owner_sex.DefaultIfEmpty()
          where oa.TypeId == 1 && oc.TypeId ==2 && os.TypeId ==3
          select new Owner() {OwnerId = owner.OwnerId, 
                              Name = owner.Name,
                              Area = oa.TypeValue,
                              City = oc.TypeValue,
                              Sex = os.TypeValue}).Distinct(); 
}

此查询有几个问题：

它返回多个结果，而distinct不会按预期工作
我曾尝试使用GroupBy，但它说不能隐式将所有者转换为IEnumerable <int, Owner>
超级慢

如何通过自我加入获得独特的记录并提高性能？感谢

更新谢谢你们为你的ansewers，现在测试，但我发现我忘了再提供一件东西了。我在表格布局中添加了一个名为TypeCodeId的新列（参见上文）用户可以根据他们的选择过滤值。所以，我有区域，城市和性别的TypeCodeId + TypeValue词典。所有这些参数都是可选的（如果用户没有选择任何参数，我只会向他们显示所有记录。

因此，假设用户已选择过滤器Area: Unites States并过滤City: Los Angeles

他们我的查询看起来像这样：

Select Projects where Area equals United States(440) and City equals Los Angeles(221)

如果选择了“仅区域：墨西哥”，那么我的查询将会显示如下：

Select Projects where Area equals Mexico(321)

我不确定如何使用您在示例中提供的内容中的可选where子句。

Answer 1

由于表未规范化，我们需要从对象/表中获取distict用户。这可以通过以下方式完成：

owners.Select(o => new { o.OwnerId, o.OwnerName }).Distinct()

然后我们需要使用两个匹配值来加入“类型”，一个用于ownerId，另一个用于特定类型。

var ownerQuery =
    from o in owners.Select(o => new { o.OwnerId, o.OwnerName }).Distinct()

    join area in owners on new { o.OwnerId, TypeId = 1 } equals new { area.OwnerId, area.TypeId } into areas
    from area in areas.DefaultIfEmpty()

    join city in owners on new { o.OwnerId, TypeId = 2 } equals new { city.OwnerId, city.TypeId } into cities
    from city in cities.DefaultIfEmpty()

    join sex in owners on new { o.OwnerId, TypeId = 3 } equals new { sex.OwnerId, sex.TypeId } into sexes
    from sex in sexes.DefaultIfEmpty()

    select new 
        { 
            owner = o,
            Area = (area != null) ? area.TypeValue : null, 
            City = (city != null) ? city.TypeValue : null, 
            Sex = (sex != null) ? sex.TypeValue : null, 
        };

您可能需要更改上例中的投影。

Answer 2

为了获得最佳性能，请认为这是实现这一目标的方法。

public IEnumerable<Owner> GetOwners(IEnumerable<Tuple<int, int>> filter)
{
    var q = (from o in owners
            join f in filter on 
               new {o.TypeId, o.TypeCodeId} equals 
               new {TypeId = f.Item1, TypeCodeId = f.Item2}
            select o).ToList();

    var dic = q.ToDictionary (o => new {o.OwnerId, o.TypeId}, o => o.TypeValue);
    foreach (var o in q.Select(o => new { o.OwnerId, o.OwnerName }).Distinct())
    {
        var owner = new Owner()
        {
            OwnerId = o.OwnerId,
            Name = o.OwnerName
        };
        string lookup;
        if(dic.TryGetValue(new {o.OwnerId, TypeId = 1}, out lookup))
            owner.Area = lookup;
        if(dic.TryGetValue(new {o.OwnerId, TypeId = 2}, out lookup))
            owner.City = lookup;
        if(dic.TryGetValue(new {o.OwnerId, TypeId = 3}, out lookup))
            owner.Sex = lookup;

        yield return owner;
    }
}

为了获得更高的性能，您可以编写一个IEqualityComparer类，仅比较int OwnerId并将其发送到Distinct函数

Linq自加入查询

2 个答案: