See The Whole Data in DataGridView With Selecting A Value From ComboBox:

*Write this code on the Button*

private void button1_Click(object sender, EventArgs e)

        {

            SqlConnection con = new SqlConnection();

            con.ConnectionString = "Data Source=home-1117080bc2;Initial Catalog=Hotel_Booking;User ID=sa;Password=sqlforarijit";

            con.Open();

            SqlCommand cmd = new SqlCommand("select * from new_reservation where customer_id = @a", con);

            cmd.Parameters.Add("@a", SqlDbType.VarChar, 50);

            cmd.Parameters["@a"].Value = comboBox1.Text;

            SqlDataAdapter ada = new SqlDataAdapter(cmd);

            SqlCommandBuilder build = new SqlCommandBuilder(ada);

            DataSet ds = new DataSet();

            ada.Fill(ds, "new_reservation");

            dataGridView1.DataSource = ds.Tables[0];

        }

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