How to create a child heading under parent heading in html table?

I want to make this template with html tables,

This is my html code

<table>
    <tr>
        <td>data</td>
        <td>data</td>
        <td>data</td>
        <td>data</td>
        <td>data</td>
        <td>data</td>
        <td>
            <table>
                <tr>
                    <td>data</td>
                    <td>data</td>
                </tr>
            </table>
        </td>
        <td>
            <table>
                <tr>
                    <td>data</td>
                    <td>data</td>
                </tr>
            </table>
        </td>
        <td>data</td>
    </tr>
</table>

But it is not giving me the right format like the image above. How can i do it with table?