我有一份人们的身份证和名字的名单,还有一份人们的身份证和姓氏的名单。有些人没有名字,有些人没有姓;我想在这两个列表上做一个完整的外部连接。
下面列出:
ID FirstName
-- ---------
1 John
2 Sue
ID LastName
-- --------
1 Doe
3 Smith
应该生产:
ID FirstName LastName
-- --------- --------
1 John Doe
2 Sue
3 Smith
我已经发现了相当多的解决方案的“LINQ外部连接”,它们看起来都很相似,但似乎真的是离开外部连接。
到目前为止我的尝试是这样的:
private void OuterJoinTest()
{
List<FirstName> firstNames = new List<FirstName>();
firstNames.Add(new FirstName { ID = 1, Name = "John" });
firstNames.Add(new FirstName { ID = 2, Name = "Sue" });
List<LastName> lastNames = new List<LastName>();
lastNames.Add(new LastName { ID = 1, Name = "Doe" });
lastNames.Add(new LastName { ID = 3, Name = "Smith" });
var outerJoin = from first in firstNames
join last in lastNames
on first.ID equals last.ID
into temp
from last in temp.DefaultIfEmpty()
select new
{
id = first != null ? first.ID : last.ID,
firstname = first != null ? first.Name : string.Empty,
surname = last != null ? last.Name : string.Empty
};
}
}
public class FirstName
{
public int ID;
public string Name;
}
public class LastName
{
public int ID;
public string Name;
}
但结果是:
ID FirstName LastName
-- --------- --------
1 John Doe
2 Sue
我做错了什么?
两个或多个表的完全外部连接:
首先提取要连接的列。
var DatesA = from A in db.T1 select A.Date;
var DatesB = from B in db.T2 select B.Date;
var DatesC = from C in db.T3 select C.Date;
var Dates = DatesA.Union(DatesB).Union(DatesC);
然后在提取的列和主表之间使用左外连接。
var Full_Outer_Join =
(from A in Dates
join B in db.T1
on A equals B.Date into AB
from ab in AB.DefaultIfEmpty()
join C in db.T2
on A equals C.Date into ABC
from abc in ABC.DefaultIfEmpty()
join D in db.T3
on A equals D.Date into ABCD
from abcd in ABCD.DefaultIfEmpty()
select new { A, ab, abc, abcd })
.AsEnumerable();
谢谢大家的有趣帖子!
我修改了代码,因为在我的情况下我需要
个性化的连接谓词
一个个性化的联合,独特的比较
这是我修改过的代码(不好意思,用VB写的)
Module MyExtensions
<Extension()>
Friend Function FullOuterJoin(Of TA, TB, TResult)(ByVal a As IEnumerable(Of TA), ByVal b As IEnumerable(Of TB), ByVal joinPredicate As Func(Of TA, TB, Boolean), ByVal projection As Func(Of TA, TB, TResult), ByVal comparer As IEqualityComparer(Of TResult)) As IEnumerable(Of TResult)
Dim joinL =
From xa In a
From xb In b.Where(Function(x) joinPredicate(xa, x)).DefaultIfEmpty()
Select projection(xa, xb)
Dim joinR =
From xb In b
From xa In a.Where(Function(x) joinPredicate(x, xb)).DefaultIfEmpty()
Select projection(xa, xb)
Return joinL.Union(joinR, comparer)
End Function
End Module
Dim fullOuterJoin = lefts.FullOuterJoin(
rights,
Function(left, right) left.Code = right.Code And (left.Amount [...] Or left.Description.Contains [...]),
Function(left, right) New CompareResult(left, right),
New MyEqualityComparer
)
Public Class MyEqualityComparer
Implements IEqualityComparer(Of CompareResult)
Private Function GetMsg(obj As CompareResult) As String
Dim msg As String = ""
msg &= obj.Code & "_"
[...]
Return msg
End Function
Public Overloads Function Equals(x As CompareResult, y As CompareResult) As Boolean Implements IEqualityComparer(Of CompareResult).Equals
Return Me.GetMsg(x) = Me.GetMsg(y)
End Function
Public Overloads Function GetHashCode(obj As CompareResult) As Integer Implements IEqualityComparer(Of CompareResult).GetHashCode
Return Me.GetMsg(obj).GetHashCode
End Function
End Class
两个或多个表的完全外部连接:
首先提取要连接的列。
var DatesA = from A in db.T1 select A.Date;
var DatesB = from B in db.T2 select B.Date;
var DatesC = from C in db.T3 select C.Date;
var Dates = DatesA.Union(DatesB).Union(DatesC);
然后在提取的列和主表之间使用左外连接。
var Full_Outer_Join =
(from A in Dates
join B in db.T1
on A equals B.Date into AB
from ab in AB.DefaultIfEmpty()
join C in db.T2
on A equals C.Date into ABC
from abc in ABC.DefaultIfEmpty()
join D in db.T3
on A equals D.Date into ABCD
from abcd in ABCD.DefaultIfEmpty()
select new { A, ab, abc, abcd })
.AsEnumerable();
下面是一个扩展方法:
public static IEnumerable<KeyValuePair<TLeft, TRight>> FullOuterJoin<TLeft, TRight>(this IEnumerable<TLeft> leftItems, Func<TLeft, object> leftIdSelector, IEnumerable<TRight> rightItems, Func<TRight, object> rightIdSelector)
{
var leftOuterJoin = from left in leftItems
join right in rightItems on leftIdSelector(left) equals rightIdSelector(right) into temp
from right in temp.DefaultIfEmpty()
select new { left, right };
var rightOuterJoin = from right in rightItems
join left in leftItems on rightIdSelector(right) equals leftIdSelector(left) into temp
from left in temp.DefaultIfEmpty()
select new { left, right };
var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin);
return fullOuterJoin.Select(x => new KeyValuePair<TLeft, TRight>(x.left, x.right));
}
我认为LINQ join子句并不是这个问题的正确解决方案,因为join子句的目的并不是按照这个任务解决方案所需的方式来积累数据。合并创建的独立集合的代码变得太复杂了,也许这对于学习目的来说是可以的,但对于真正的应用程序来说不是。解决这个问题的方法之一是下面的代码:
class Program
{
static void Main(string[] args)
{
List<FirstName> firstNames = new List<FirstName>();
firstNames.Add(new FirstName { ID = 1, Name = "John" });
firstNames.Add(new FirstName { ID = 2, Name = "Sue" });
List<LastName> lastNames = new List<LastName>();
lastNames.Add(new LastName { ID = 1, Name = "Doe" });
lastNames.Add(new LastName { ID = 3, Name = "Smith" });
HashSet<int> ids = new HashSet<int>();
foreach (var name in firstNames)
{
ids.Add(name.ID);
}
foreach (var name in lastNames)
{
ids.Add(name.ID);
}
List<FullName> fullNames = new List<FullName>();
foreach (int id in ids)
{
FullName fullName = new FullName();
fullName.ID = id;
FirstName firstName = firstNames.Find(f => f.ID == id);
fullName.FirstName = firstName != null ? firstName.Name : string.Empty;
LastName lastName = lastNames.Find(l => l.ID == id);
fullName.LastName = lastName != null ? lastName.Name : string.Empty;
fullNames.Add(fullName);
}
}
}
public class FirstName
{
public int ID;
public string Name;
}
public class LastName
{
public int ID;
public string Name;
}
class FullName
{
public int ID;
public string FirstName;
public string LastName;
}
如果真正的集合对于HashSet的形成很大,而不是foreach循环可以使用下面的代码:
List<int> firstIds = firstNames.Select(f => f.ID).ToList();
List<int> LastIds = lastNames.Select(l => l.ID).ToList();
HashSet<int> ids = new HashSet<int>(firstIds.Union(LastIds));//Only unique IDs will be included in HashSet