使用 select from 从表中获取值以存储在插入表中答案

【问题标题】：Get values from a table with select from to be stored in insert into table使用 select from 从表中获取值以存储在插入表中
【发布时间】：2020-08-11 18:33:27
【问题描述】：

我有一个项目有两个窗口，其中一个是登录页面，SQL 设置为select from，另一个页面是学生，设置为insert into。

我希望表 student 中的列 username 以某种方式出现在插入表中。

public string saveduser;

private void button_Click(object sender, RoutedEventArgs e)
{
    SqlConnection sqlCon = new SqlConnection(@"Data Source = root\SQLEXPRESS; Initial Catalog =Log-In; Integrated Security = True");

    try
    {
        if (sqlCon.State == ConnectionState.Closed)
            sqlCon.Open();

        string query = "SELECT COUNT(1) FROM Login WHERE Username = @Username AND password = @password AND usertype = @usertype";

        SqlCommand sqlCmd = new SqlCommand(query, sqlCon);
        sqlCmd.CommandType = CommandType.Text;

        sqlCmd.Parameters.AddWithValue("@Username", txtUsername.Text);
        sqlCmd.Parameters.AddWithValue("@password", txtPassword.Text);
    }
    ......
}

学生页面：

private void button_Click(object sender, RoutedEventArgs e)
{
    Login aa = new Login();
    string newsaveduser = aa.saveduser;

    using (var Con = new SqlConnection(@"Data Source=root\SQLEXPRESS; Initial Catalog=Log-In; Integrated Security=True"))
    {
        Con.Open();

        var table = "INSERT INTO  Student (username, DepartureCity, ArrivalCity, DateOfDeparture, DateOfReturn) VALUES (@username, @DepartureCity, @ArrivalCity, @DateOfDeparture, @DateOfReturn)";

        using (var cmd = new SqlCommand(table, Con))
        {
            // again here trying to use the textbox but it keep giving me error  SqlParameter unitsParam = cmd.Parameters.AddWithValue("@username", newsaveduser);
            if (aa.saveduser == null)
            {
                unitsParam.Value = DBNull.Value;
            }

            cmd.Parameters.AddWithValue("@DepartureCity", fromtxt.Text);
            cmd.Parameters.AddWithValue("@ArrivalCity", Totxt.Text);
            cmd.Parameters.AddWithValue("@DateOfDeparture", date_time_picker.Text);
            cmd.Parameters.AddWithValue("@DateOfReturn", date_time_picker1.Text);

            cmd.ExecuteNonQuery();

            MessageBox.Show("Data inserted successfully");
        }
    }
}

我在这段代码中遇到的错误：

附加信息：无法将值 NULL 插入“用户名”列、“Log-In.dbo.Student”表；列不允许空值。插入失败

【问题讨论】：

cmd.Parameters.AddWithValue("@username", your_userName);

标签： c# sql-server wpf

【解决方案1】：

您的学生页面上似乎忘记添加用户名

cmd.Parameters.AddWithValue("@Username", txtUsername.Text);
cmd.Parameters.AddWithValue("@DepartureCiry", fromtxt.Text);
cmd.Parameters.AddWithValue("@ArrivalCity", Totxt.Text);
cmd.Parameters.AddWithValue("@DateOfDeparture", date_time_picker.Text);
cmd.Parameters.AddWithValue("@DateOfReturn", date_time_picker1.Text);

【讨论】：