Grid View Delete Record from table in SQL DB

protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
    string id = GridView1.DataKeys[e.RowIndex].Values["Id"].ToString();
    SqlCommand cmd = new SqlCommand();
    cmd.CommandText = "Delete FROM TableName where Id='" + id + "'";
    cmd.Connection = con;
    con.Open();
    cmd.ExecuteNonQuery();
    con.Close();
    GetGridData();
    Response.Write("<script>alert('Module Deleted..!!!');</script>");
}

and on aspx page

<asp:GridView ID="GridView1" runat="server" CellPadding="4" ForeColor="#333333" 
            GridLines="None" AutoGenerateColumns="False"
            DataKeyNames="Id" AllowPaging="True"                 
            onrowdeleting="GridView1_RowDeleting">   
            <Columns>
             <asp:TemplateField HeaderText="Id">
                    <ItemTemplate>
                        <asp:Label ID="lblID" runat="server" Text='<%# Bind("Id") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>                               
                // here your other colums                   
                <asp:TemplateField HeaderText="Delete?">
                    <ItemTemplate>
                        <span onclick="return confirm('Are you sure to Delete the record?')">
                            <asp:LinkButton ID="lnkB" runat="Server" Text="Delete" CommandName="Delete"></asp:LinkButton>
                        </span>
                    </ItemTemplate>
                </asp:TemplateField>
    </Columns>          
            <AlternatingRowStyle BackColor="White" />
            <EditRowStyle BackColor="#2461BF" />
            <FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
            <HeaderStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
            <RowStyle BackColor="#EFF3FB" />
            <SelectedRowStyle BackColor="#D1DDF1" Font-Bold="True" ForeColor="#333333" />
            <sortedascendingcellstyle backcolor="#F5F7FB" />
            <sortedascendingheaderstyle backcolor="#6D95E1" />
            <sorteddescendingcellstyle backcolor="#E9EBEF" />
            <sorteddescendingheaderstyle backcolor="#4870BE" />
        </asp:GridView>

And for reference use that link:

Edit,update, delete gridview

Can you add command link for delete in the grid for each row

A quick googling gave me this tutorial
http://www.c-sharpcorner.com/UploadFile/9f0ae2/gridview-edit-delete-and-update-in-Asp-Net/

If you are using Template field,

 protected void GridView1_RowDeleting(object sender, GridViewEditEventArgs e)
{  // you can retrieve the value of each row like
  Label lblRequestID=(Label)GridView1.Rows[e.NewEditIndex].FindControl("ControlID");
   Delete(lblRequestID.Text);
}

// you can use below code if u r using stored procedure

 public void Delete(string id)
    {

        CMD.Parameters.Clear();
        CMD.CommandText = "SP name";
        CMD.Parameters.AddWithValue("@ID", id);
        try
        {
            CON.Open();
            CMD.ExecuteReader();
        }
        catch (Exception ex)
        {
            Console.Write(ex.InnerException);
        }

        finally
        {
            CON.Close();
        }
    }

Your example code does not have valid T-SQL. Try this:

GridViewRow row = LeaveGrid.SelectedRow; 
DBConn dbcon = new DBConn();
int idToDelete = Convert.ToInt32(LeaveGrid.DataKeys[e.RowIndex].Value.ToString());
String qry = "Delete from Leave where id=" + idToDelete;