C#

MyDataAdapter.Fill(MyDataTable);
MyMasterDGV.DataSource = myDataTable;
MyDetailDGV.DataSource = myDataTable;

private string connectionString = "Data Source=DESKTOP-5GRDQ7C\\SQLEXPRESS;Initial Catalog=Orders;Integrated Security=True"; 
        private SqlDataAdapter masterDataAdapter;
        private SqlDataAdapter detailDataAdapter;
        private DataSet dataSet;

        public Form1()
        {
            InitializeComponent();
        }

        
        private void LoadMasterData()
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                string query = "SELECT * FROM ordm";
                SqlDataAdapter adapter = new SqlDataAdapter(query, connection);
                DataTable dataTable = new DataTable();
                adapter.Fill(dataTable);

               
                masterDataGridView.DataSource = dataTable;
            }
        }
        private void LoadData()
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
           
                string ordmQuery = "SELECT * FROM ORDM";
                masterDataAdapter = new SqlDataAdapter(ordmQuery, connection);
                masterDataAdapter.Fill(dataSet, "ORDM");

              
                string ordcQuery = "SELECT * FROM ORDC";
                detailDataAdapter = new SqlDataAdapter(ordcQuery, connection);
                detailDataAdapter.Fill(dataSet, "ORDC");
            }

            
            DataColumn parentColumn = dataSet.Tables["ORDM"].Columns["ORD_NO"];
            DataColumn childColumn = dataSet.Tables["ORDC"].Columns["ORD_NO"];
            DataRelation relation = new DataRelation("ORDM_ORDC_Relation", parentColumn, childColumn);
            dataSet.Relations.Add(relation);
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            LoadMasterData();
           
            dataSet = new DataSet();
            LoadData();

           
            masterDataGridView.DataSource = dataSet.Tables["ORDM"];

            
            masterDataGridView.SelectionChanged += masterDataGridView_SelectionChanged_1;
        }

        private void masterDataGridView_SelectionChanged_1(object sender, EventArgs e)
        {
           
            if (masterDataGridView.SelectedRows.Count > 0)
            {
              
                string orderNumber = Convert.ToString(masterDataGridView.SelectedRows[0].Cells["ordcol"].Value);

              
                LoadChildData(orderNumber);
            }
        }
            private void LoadChildData(string orderNumber)
            {
                using (SqlConnection connection = new SqlConnection(connectionString))
                {
                   
                    string query = $"SELECT * FROM ordc WHERE ord_no = ord_no";
                    SqlDataAdapter adapter = new SqlDataAdapter(query, connection);
                    DataTable dataTable = new DataTable();
                    adapter.Fill(dataTable);

                   
                    childDataGridView.DataSource = dataTable;
                }
            }
    }

string query = $"SELECT * FROM ordc WHERE ord_no = ord_no";

private void CreateTextBox(string i) // contains "ltIndexN", where N is the next index number
{

    Literal ltIndex = new Literal();
    ltIndex.Text = 1 + ".";            // but here you always create the same value
    pnlTextBoxes.Controls.Add(ltIndex); // you should use the passed in string here
    Literal lt = new Literal();
    lt.Text = "<br>";
    pnlTextBoxes.Controls.Add(lt);
}

public static DataTable GetAll()
{
    const string Query = """
        SELECT
            ...
        FROM
            ...
        """;

    try
    {
        using (var connection = Connection.CreateAndOpenConnection()) // Make this method open the connection before returning it...
        using (var command = new SqlCommand(connection, Query))
        using (var reader = command.ExecuteReader())
        {
            DataTable dt = new DataTable();
            dt.Load(reader);
            return dt;
        }
    }
    catch (SqlException ex)
    {
        throw new YourCustomException("There was an error retrieving the list of all things. See the inner exception for details.", ex);
    }
}

Quote:

Maximum Number Of Host Parameters In A Single SQL Statement

A host parameter is a place-holder in an SQL statement that is filled in using one of the sqlite3_bind_XXXX() interfaces. Many SQL programmers are familiar with using a question mark ("?") as a host parameter. SQLite also supports named host parameters prefaced by ":", "$", or "@" and numbered host parameters of the form "?123".

Each host parameter in an SQLite statement is assigned a number. The numbers normally begin with 1 and increase by one with each new parameter. However, when the "?123" form is used, the host parameter number is the number that follows the question mark.

SQLite allocates space to hold all host parameters between 1 and the largest host parameter number used. Hence, an SQL statement that contains a host parameter like ?1000000000 would require gigabytes of storage. This could easily overwhelm the resources of the host machine. To prevent excessive memory allocations, the maximum value of a host parameter number is SQLITE_MAX_VARIABLE_NUMBER, which defaults to 999 for SQLite versions prior to 3.32.0 (2020-05-22) or 32766 for SQLite versions after 3.32.0.

The maximum host parameter number can be lowered at run-time using the sqlite3_limit(db,SQLITE_LIMIT_VARIABLE_NUMBER,size) interface.