VB.Net LINQ - 两个数据表之间的左外连接 - 限制为一行 [英] VB.Net LINQ - left outer join between two datatables - limit to one row

问题描述

我在 vb.net 中遇到了 LINQ 问题.基本上我想在两个数据表之间创建 LEFT JOIN.

I have an issue with LINQ in vb.net. Basically I want create LEFT JOIN between two datatables.

这是两个数据表的信息:

This is the info of the two datatables:

Dim vDT1 As New DataTable
vDT1.Columns.Add("Key")
vDT1.Columns.Add("Data1")
vDT1.Columns.Add("Data2")

vDT1.Rows.Add({"01", "DATA1_AAAA", "DATA2_AAAA"})
vDT1.Rows.Add({"02", "DATA1_BBBB", "DATA2_BBBB"})

Dim vDT2 As New DataTable
vDT2.Columns.Add("Key")
vDT2.Columns.Add("Data3")
vDT2.Columns.Add("Data4")

vDT2.Rows.Add({"01", "DATA3_AAAA", "DATA4_AAAA"})
vDT2.Rows.Add({"01", "DATA3_BBBB", "DATA4_BBBB"})
vDT2.Rows.Add({"01", "DATA3_CCCC", "DATA4_CCCC"})
vDT2.Rows.Add({"01", "DATA3_DDDD", "DATA4_DDDD"})

Dim vDRnull As DataRow = vDTsec.Rows.Add

为了实现LINQ JOIN我尝试使用下一句:

In order to achieve the LINQ JOIN I try to use the next sentence:

From a In vDT1.AsEnumerable
Group Join bTemp In vDT2.AsEnumerable
On a.Field(Of String)("Key") Equals bTemp.Field(Of String)("Key")
Into Group From b In Group.DefaultIfEmpty(vDRnull)
Select a, b

这是结果(但我想删除红色行)，我只想在第二个表中第一次出现:

This is the result (but I want remove the red rows), I only want get the first occurrence in the second table:

任何帮助将不胜感激！

我用正确的方法编辑了问题.感谢杰夫·梅尔卡多:

I edit the question with the correct method. Thanks to Jeff Mercado:

Dim vDT1 As New DataTable
vDT1.Columns.Add("Key")
vDT1.Columns.Add("Data1")
vDT1.Columns.Add("Data2")

vDT1.Rows.Add({"01", "DATA1_AAAA", "DATA2_AAAA"})
vDT1.Rows.Add({"02", "DATA1_BBBB", "DATA2_BBBB"})

Dim vDT2 As New DataTable
vDT2.Columns.Add("Key")
vDT2.Columns.Add("Data3")
vDT2.Columns.Add("Data4")

vDT2.Rows.Add({"01", "DATA3_AAAA", "DATA4_AAAA"})
vDT2.Rows.Add({"01", "DATA3_BBBB", "DATA4_BBBB"})
vDT2.Rows.Add({"01", "DATA3_CCCC", "DATA4_CCCC"})
vDT2.Rows.Add({"01", "DATA3_DDDD", "DATA4_DDDD"})

Dim vDRnull As DataRow = vDT2.Rows.Add

Dim vLINQ = From a In vDT1.AsEnumerable
    Group Join bTemp In vDT2.AsEnumerable
    On a.Field(Of Object)("Key") Equals bTemp.Field(Of Object)("Key")
    Into Group Let b = If(Group.FirstOrDefault Is Nothing, vDRnull, Group.FirstOrDefault)
    Select a, b

推荐答案

关键是对组调用FirstOrDefault().那么您要么拥有组中的第一行，要么Nothing.

The key is to call FirstOrDefault() on the group. Then you either have the first row in the group or Nothing.

Dim query =
    From a In vDT1.AsEnumerable
    Group Join b In vDT2.AsEnumerable
        On a.Field(Of String)("Key") Equals b.Field(Of String)("Key")
        Into Group
    Let b = Group.FirstOrDefault
    Select Key = a.Field(Of String)("Key"),
           Data1 = a.Field(Of String)("Data1"),
           Data2 = a.Field(Of String)("Data2"),
           Data3 = If(b Is Nothing, Nothing, b.Field(Of String)("Data3")),
           Data4 = If(b Is Nothing, Nothing, b.Field(Of String)("Data4"))

    ' Or Simply
    'Select a, b

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