如何使用命令从 SQL 查询中获取单行/记录。VB.NET 中的 ExecuteScalar-解网

问：

我试图通过执行带有主键的SELECT语句来获取一行

我尝试将输出分配给变量，然后尝试通过键访问值，例如command.ExecuteScalar()Objectobj('column_name')

Using conn As New SQLiteConnection(connectionString:=connection.get_connection_string())
                    conn.Open()
    Dim sql_string As String = "SELECT * FROM employees WHERE id = @id"
    Using cmd As New SQLiteCommand(connection:=conn, commandText:=sql_string)
        cmd.Parameters.AddWithValue("@id", employee_id)
        Dim reader As Object = cmd.ExecuteScalar()
        If reader <> Nothing Then
            Me.user_details(2) = reader("first_name")
        End If
    End Using
End Using

我本来希望从中获取列中的值，但我得到了一个异常first_namereader('first_name')NullReferenceException

vb.net sqlite ado.net executescalar

Dim sql_string As String = "SELECT first_name FROM employees WHERE id = @id"
Using cmd As New SQLiteCommand(connection:=conn, commandText:=sql_string)
    cmd.Parameters.AddWithValue("@id", employee_id)
    Dim result As Object = cmd.ExecuteScalar()
    If result <> Nothing Then
        Me.user_details(2) = result.ToString()
    End If
End Using

相反，如果要从该记录中检索所有字段（或多个字段），请使用 ExecuteReader，然后开始读取结果

Dim sql_string As String = "SELECT first_name, last_name, city FROM employees WHERE id = @id"
Using cmd As New SQLiteCommand(connection:=conn, commandText:=sql_string)
    cmd.Parameters.AddWithValue("@id", employee_id)
    Dim reader = cmd.ExecuteReader()
    ' Try to position the reader on the first record returned by the query
    ' if there aren't records the Read returns false. 
    ' You can also test this with reader.HasRows
    While reader.Read()
        Me.user_details(1) = reader("last_name").ToString()
        Me.user_details(2) = reader("first_name").ToString()
        Me.user_details(3) = reader("city").ToString()
    End While
End Using

如何使用命令从 SQL 查询中获取单行/记录。VB.NET 中的 ExecuteScalar

How to get single row/record from a SQL query using command.ExecuteScalar in VB.NET

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