How to store multiple items from checkbox list into the database ?

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C#

SQL-Server

Hello everyone,

Well i'm developing a library system for students, in which I need to provide TAGS to the books, in the ADD book section, where I've used a CHECKBOX LIST (because tags can be multiple) FOR THE PREDETERMINED TAGS, and then a TEXT BOX to add CUSTOM TAGS. But I'm not able to store them with a "," separation in between into the database.

Here's the piece of code I'm using:

protected void butt_addbook_Click(object sender, EventArgs e)
   {


     SqlConnection con=new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
     con.Open();
       string tag = "";
     String query = "insert into demo_book_info values ('" + txt_bookno.Text + "','" + txt_title.Text + "','" + txt_bookauthor.Text + "','"+ tag +"','" + txt_bookcopies.Text + "','Available')";

     try
     {
         SqlCommand cmd = new SqlCommand(query, con);



         foreach (ListItem li in checkboxlist_tags.Items)
         {
             if(li.Selected==true)
             {
                 tag += "," + li.Text;
             }

         }
         tag += "," + txt_custom_tags.Text;



         cmd.ExecuteNonQuery();
         ClientScript.RegisterClientScriptBlock(Page.GetType(),"validation",
         "<script language='javascript'>alert ('Saving is Done!')</script>");
         con.Close();
     }
     catch (Exception ex)
     {
         Response.Write(ex.Message);
         ClientScript.RegisterStartupScript(Page.GetType(), "validation",
             "<script language='javascript'>alert('Error!')</script>");
     }