如何检查文本框中的值是否已存在于连接的数据库中

Question

所以我目前对我的程序的逻辑感到困惑，我已经多次运行它但我不太确定。我基本上希望能够通知用户说他们输入的联系电话已经存在于数据库中。我已经接近了，我确信我已经做了很多研究，但我不认为我所做的实际上是检查数据库，而只是告诉它匹配文本框中的任何内容，而不是检查数据库。

只是为了澄清我不想从数据库中提取数据并将其放入文本框中。谢谢。

更新：针对两种不同建议解决方案的两种不同程序。

（可能的解决方案 1）更新的联系电话是使用 COUNT 的唯一方法：

private void CheckContactNumber()
{
    string checkContactNum = "SELECT COUNT(*) FROM Employee WHERE ContactNumber = " + addContactNum.Text + " "; //01234567890

    OleDbCommand cmd = new OleDbCommand(checkContactNum, conn);
    conn.Open();
    OleDbDataReader dr = cmd.ExecuteReader();

    if (dr.Read())
    {
        int countDup = (int)dr[0];

        if (countDup > 0 && addContactNum.Text != "")
        {
            err += "Contact number is already listed in the database\r\n";
            errorContactNum.Visible = true;
            uniqueContactNumber = false;
        }
        else if (countDup == 0 && addContactNum.Text != "")
        {
            errorContactNum.Visible = false;
            uniqueContactNumber = true;
        }
    }

    conn.Close();
}

（可能的解决方案 2）更新到建议的第二个解决方案：

if (err == "")
{
    // you miss s here
    string addEmployee = "if not exists(select LastName from Employee where LastName = @LastName)INSERT INTO Employee(FirstName, LastName, Role, DateOfHire, ContactNumber)" +"VALUES(@FirstName, @LastName, @Role, @DateOfHire, @ContactNumber)";

    OleDbCommand cmd = new OleDbCommand(addEmployee, conn);
    OleDbDataAdapter da = new OleDbDataAdapter(cmd);
    cmd.Parameters.Add("@FirstName", OleDbType.VarChar).Value = addFirstName.Text;
    cmd.Parameters.Add("@LastName", OleDbType.VarChar).Value = addLastName.Text;
    cmd.Parameters.Add("@Role", OleDbType.VarChar).Value = addRole.Text;
    cmd.Parameters.Add("@DateOfHire", OleDbType.VarChar).Value = addDateOfHire.Text;
    cmd.Parameters.Add("@ContactNumber", OleDbType.VarChar).Value = addContactNum.Text;

    conn.Open();

    // i removed the duplicated code
    if (cmd.ExecuteNonQuery() != 1)
    {
        err += "Contact number is already listed in the database\r\n";
                errorContactNum.Visible = true;
    }

    conn.Close();

    addFirstName.Text = String.Empty;
    addLastName.Text = String.Empty;
    addRole.Text = String.Empty;
    addContactNum.Text = String.Empty;
    addRole.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
    }
    //Save It
    else
    {
        MessageBox.Show(err);
    }
}

So this would be acceptable for adding to my database as all criterion is met but it doesnt accept it, I'm assuming it runs the else code right at the bottom of AddEmployee method

Here is an Image of my database which is linked to my program.

Answer 1

从头到尾更新你的功能

private void CheckContactNumber()
    {

        string checkContactNum = "SELECT COUNT(*) FROM Employee WHERE ContactNumber = " + addContactNum.Text + " "; //01234567890

        OleDbCommand cmd = new OleDbCommand(checkContactNum, conn);
        conn.Open();
        OleDbDataReader dr = cmd.ExecuteReader();

        //if (dr.Read() && addContactNum.Text != "")
        if (dr.Read())
        {
            int count = (int)dr[0];
            if(count>0)
            {
                err += "Contact number is already listed in the database\r\n";
                errorContactNum.Visible = true;
                uniqueContactNumber = false;
            }

        }

        conn.Close();

    }

更新答案

    private void CheckContactNumber()
    {

        DataSet myDataSet = new DataSet();


        try
        {
            string strAccessSelect = "select count(*) from Employee where ContactNumber='" + addContactNum.Text + "'";
            OleDbCommand myAccessCommand = new OleDbCommand(strAccessSelect, conn);
            OleDbDataAdapter myDataAdapter = new OleDbDataAdapter(myAccessCommand);

            conn.Open();
            myDataAdapter.Fill(myDataSet, "Employee");


        }
        catch (Exception ex)
        {
            Console.WriteLine("Error: Failed to retrieve the required data from the DataBase.\n{0}", ex.Message);
            return;
        }
        finally
        {
            conn.Close();
        }
        DataTable dt = myDataSet.Tables[0];
        if (dt != null)
        {
            if (int.Parse(dt.Rows[0][0].ToString()) > 0)
            {
                string err = "Contact Number Already exist..";
            }
        }
    }

Answer 2

你可以将两个代码合并到一个代码中，像这样在 sql server 中使用 if not exist 方法

string addEmployee = "if not exists(select LastName from Employee where 
LastName=@LastName)INSERT INTO Employee (FirstName, LastName, Role, 
DateOfHire, ContactNumber)" +"VALUES (@FirstName, @LastName, @Role, @DateOfHire, @ContactNumber)";

if (cmd.ExecuteNonQuery()!=1){
// the employee exist in database
}