如何檢查ID是否已存儲在數據庫中？

Question

我有將此代碼插入數據庫的代碼。 我想檢查要插入的ID是否已在數據庫中。 但是，如果它不在數據庫中，則應將其插入。 但是有一些錯誤，您能幫我解決嗎？

    public void Add()
    {
        sc.Open();
        try
        {
            cmd = new SqlCommand("Select idnum from TableVotersInfo Where idnum=@idnum", sc);

            cmd.Parameters.AddWithValue("@idnum", _idnum);

            SqlDataReader rd = cmd.ExecuteReader();
            if (rd.Read() == true)
            {
                MessageBox.Show("ID number already exist!");
                rd.Close();
            }
            else
            {
                cmd = new SqlCommand("INSERT INTO TableVotersInfo (Education, idnum, FirstName, MiddleName, LastName, SchoolYear, ControlNum, VResult) VALUES (@ed, @idnum, @firstname, @middlename, @lastname, @schoolyear, @controlnum, 'Not Voted');", sc);
                cmd.Parameters.AddWithValue("@id", _id);
                cmd.Parameters.AddWithValue("@ed", _ed);
                cmd.Parameters.AddWithValue("@idnum", _idnum);
                cmd.Parameters.AddWithValue("@firstname", _firstname);
                cmd.Parameters.AddWithValue("@middlename", _middlename);
                cmd.Parameters.AddWithValue("@lastname", _lastname);
                cmd.Parameters.AddWithValue("@schoolyear", _schoolyear);
                cmd.Parameters.AddWithValue("@controlnum", _controlnum);

                cmd.ExecuteNonQuery();
                MessageBox.Show("Data Stored Successfully!");
                FAddVoters._cleardata = cleardata;
            }

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
        finally
        {
            sc.Close();
        }
    }

編輯我的錯誤現在是There is already an open DataReader associated with this command which must be closed first. 。

更新資料

    public void Update()
    {
        sc.Open();

        try
        {
            cmd = new SqlCommand("UPDATE TableVotersInfo SET Education=@ed, idnum=@idnum, FirstName=@firstname, MiddleName=@middlename, LastName=@lastname, SchoolYear=@schoolyear, ControlNum=@controlnum WHERE id=@id", sc);
            cmd.Parameters.AddWithValue("@id", _id);
                cmd.Parameters.AddWithValue("@ed", _ed);
                cmd.Parameters.AddWithValue("@idnum", _idnum);
                cmd.Parameters.AddWithValue("@firstname", _firstname);
                cmd.Parameters.AddWithValue("@middlename", _middlename);
                cmd.Parameters.AddWithValue("@lastname", _lastname);
                cmd.Parameters.AddWithValue("@schoolyear", _schoolyear);
                cmd.Parameters.AddWithValue("@controlnum", _controlnum);

            int res = cmd.ExecuteNonQuery();
            if (res > 0)
            {
                MessageBox.Show("Successfully Updated!");
                FAddVoters._cleardata = cleardata;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
        finally
        {
            sc.Close();
        }
    }

Answer 1

嘗試像這樣刪除查詢中的單引號

cmd = new SqlCommand("Select idnum from TableVotersInfo Where idnum=@idnum", sc);

並將變量_idnum類型從字符串更改為int

為解決插入錯誤嘗試此代碼

public void Add()
{
    sc.Open();
    try
    {
        cmd = new SqlCommand("Select idnum from TableVotersInfo Where idnum=@idnum", sc);
        cmd.Parameters.AddWithValue("@idnum", _idnum);

        if (cmd.ExecuteScalar() != null)
        {
            MessageBox.Show("ID number already exist!");
        }
        else
        {
            cmd = new SqlCommand("INSERT INTO TableVotersInfo (Education, idnum, FirstName, MiddleName, LastName, SchoolYear, ControlNum, VResult) VALUES (@ed, @idnum, @firstname, @middlename, @lastname, @schoolyear, @controlnum, 'Not Voted');", sc);
            cmd.Parameters.AddWithValue("@id", _id);
            cmd.Parameters.AddWithValue("@ed", _ed);
            cmd.Parameters.AddWithValue("@idnum", _idnum);
            cmd.Parameters.AddWithValue("@firstname", _firstname);
            cmd.Parameters.AddWithValue("@middlename", _middlename);
            cmd.Parameters.AddWithValue("@lastname", _lastname);
            cmd.Parameters.AddWithValue("@schoolyear", _schoolyear);
            cmd.Parameters.AddWithValue("@controlnum", _controlnum);

            cmd.ExecuteNonQuery();
            MessageBox.Show("Data Stored Successfully!");
            FAddVoters._cleardata = cleardata;
        }

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    finally
    {
        sc.Close();
    }
}

Answer 2

您可以通過將values更改為select並添加where子句來代替兩個查詢，而只需一次。 之后，您只需要檢查ExecuteNonQuery()的返回值即可查看是否更新了0或1行。

public void Add()
{
    sc.Open();
    try
    {
        using(cmd = new SqlCommand(@"INSERT INTO TableVotersInfo (Education, idnum, FirstName, MiddleName, LastName, SchoolYear, ControlNum, VResult)
                                        SELECT @ed, @idnum, @firstname, @middlename, @lastname, @schoolyear, @controlnum, 'Not Voted' 
                                        WHERE @idNum NOT IN (SELECT idNum FROM TableVotersInfo);", sc))
        {
            cmd.Parameters.AddWithValue("@id", _id);
            cmd.Parameters.AddWithValue("@ed", _ed);
            cmd.Parameters.AddWithValue("@idnum", _idnum);
            cmd.Parameters.AddWithValue("@firstname", _firstname);
            cmd.Parameters.AddWithValue("@middlename", _middlename);
            cmd.Parameters.AddWithValue("@lastname", _lastname);
            cmd.Parameters.AddWithValue("@schoolyear", _schoolyear);
            cmd.Parameters.AddWithValue("@controlnum", _controlnum);

            var rowsAffected = cmd.ExecuteNonQuery();
            if(rowsAffected == 0)
            {
                MessageBox.Show("ID number already exist!");
            }
            else
            {
                MessageBox.Show("Data Stored Successfully!");
                FAddVoters._cleardata = cleardata;
            }
        }

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
    finally
    {
        sc.Close();
    }
}

Answer 3

嘗試cmd.Dispose（）; 在其他{}中，然后再次初始化命令對象。 這應該適合您的情況。

Answer 4

在else也關閉datareader對象