EE template: combined index and detail page

index

...

{!-- Segment_2 isn't blank, so show a blog post's detail page --}
{if segment_2 != ""}

  <h1>Single Post</h1>

  {exp:channel:entries channel="blog" url_title="{segment_2}"}

    {!-- If segment_2 isn't a URL title for any existing blog post, redirect to the landing page --}
    {if no_results}
      {redirect="blog/index"}
    {/if}

    {!-- Otherwise display the blog post, with whatever markup you want --}

    <article class="blog_post">

      <h1>{title}</h1>
      {your_custom_blog_post_field_1}
      {your_custom_blog_post_field_2}
      {your_custom_blog_post_field_etc..}

    </article>

  {/exp:channel:entries}

{!-- Segment_2 is blank, so show the blog landing page --}
{if:else}

  <h1>All Posts</h1>

  {exp:channel:entries channel="blog" orderby="title" sort="asc"}

    <article class="blog_post">

      <h2><a href="{url_title_path='blog'}">{title}</a></h2>

    </article>

  {/exp:channel:entries}

{/if}

...