Question

我是C＃的新手，我正在编写一个桌面应用程序以读取MS-Access发票表，以便可以对其进行进一步处理。但是即使表中有行，我也无法获取“ reader.HasRows”语句来从SELECT语句返回行。每次运行该应用程序时，都会显示“未找到发票记录”消息框。

我已经在这个论坛上研究并阅读了其他类似问题的文章，但似乎没有一个能解决我的特定问题。我也在互联网上搜索了没有结果的解决方案。有人可以帮我发现我在做什么错吗？

    private void AddContactsAcconutNumberToInvoices()
    {
        //Use a variable to hold the SQL statement.
        string inputString = "SELECT Invoices.[Job_Name], * FROM Invoices " +
            "WHERE Invoices.[Account_Number] = Null";

        try
        {
            //Create an OleDbCommand object and pass in the SQL statement and OleDbConnection object
            OleDbCommand cmd = new OleDbCommand(inputString, conn);

            //Send the CommandText to the connection, and then build an OleDbDataReader.
            OleDbDataReader reader = cmd.ExecuteReader();

            // Read through the database and test the integrity of the data
            if (reader.HasRows)
            {
                while (reader.Read())
                {
                    var jobname = reader.GetString(0);
                    var contactsQuery = "SELECT Account_Number FROM CONTACTS WHERE Contacts.Full_Name = " + jobname;
                    MessageBox.Show("Contacts query contatins: " + contactsQuery);
                }
            }
            else { MessageBox.Show("No Invoice records found"); }
        }
        catch (Exception ex)
        {
            error_message = ex.Message;
            MessageBox.Show(error_message);
        }
    }

Answer 1

问题出在您的查询，尤其是= Null。您需要使用Is Null：

string inputString = "SELECT Invoices.[Job_Name], * FROM Invoices " +
        "WHERE Invoices.[Account_Number] Is Null";

不仅对于Access，而且对于SQL（该语言），这都是正确的。

此处专门针对Access：https://support.office.com/en-us/article/examples-of-query-criteria-3197228c-8684-4552-ac03-aba746fb29d8#bm1

reader.HasRows每次都返回FALSE

1 个答案: