从 VB.net 中的数据表中选择条件
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Select with condition from a datatable in VB.net
提问by HelpASisterOut
I want to select a certain field from a datatable in VB based on the value of another field in the same row.
我想根据同一行中另一个字段的值从 VB 中的数据表中选择某个字段。
In SQL, it would easily be done by writing this query:
在 SQL 中，可以通过编写以下查询轻松完成：
select error_message from table_errors where error_case="condition"
How do I do this if I have my SQL table filled in a datatable in VB? How do I select the item("error_message") in the datatable based onthe item("error_Case") field?
如果我在 VB 中的数据表中填充了我的 SQL 表，我该怎么做？如何根据item("error_Case") 字段选择数据表中的 item("error_message") ？
Any help would be appreciated
任何帮助，将不胜感激
回答by Tim Schmelter
You can use Linq-To-DataSet:
您可以使用Linq-To-DataSet：
Dim matchingRows As IEnumerable(Of DataRow) =
From row In table
Where row.Field(Of String)("error_case") = "condition"
If you just want one column (of course that works also in one step):
如果您只想要一列（当然也可以在一个步骤中使用）：
Dim errorMessages As IEnumerable(Of String) =
From row In matchingRows
Select row.Field(Of String)("error_message")
For Each error In errorMessages
Console.WriteLine(error)
Next
If you expect it to be just a single row use Firstor Single(throws an exception if there is more than one row):
如果您希望它只是一行，请使用Firstor Single（如果有多行，则抛出异常）：
Dim error As String = errorMessages.First()
Since Firstthrows an exception if the sequence is empty you can use FirstOrDefault:
由于First如果序列为空会引发异常，您可以使用FirstOrDefault：
Dim error As String = errorMessages.FirstOrDefault() ' is null/Nothing in case of an empty sequence
All in one line (note that both Linq and DataTable.Selectneeds to use loops):
全部在一行中（请注意，Linq 和都DataTable.Select需要使用循环）：
Dim ErrMessage As String = errorTable.AsEnumerable().
Where(Function(r) r.Field(Of String)("Error_Case") = TextCase.Text).
Select(Function(r) r.Field(Of String)("Error_Message")).
FirstOrDefault()
回答by Catalin
here is a worker version of the rough code
这是粗略代码的工人版本
Dim connString As String = "select error_message from table_errors where error_case='condition'"
Dim conn As SqlClient.SqlConnection = New SqlClient.SqlConnection(connString)
conn.Open()
Dim cmd As SqlClient.SqlCommand = New SqlClient.SqlCommand(connString, conn)
Dim dTable As DataTable = New DataTable()
Dim dAdapter As SqlClient.SqlDataAdapter = New SqlClient.SqlDataAdapter(cmd)
dAdapter.Fill(dTable)
conn.Close()
Dim text As String
Dim dReader As DataTableReader = dTable.CreateDataReader()
While dReader.Read()
text = dReader.GetValue(0)
End While
dReader.Close()
