lownice on "Adding URL Conditions to Featured Image via Custom Fields"

I would like to add links to a featured image on some of my single posts. These links need to be unique in a way that it adds my Amazon Affiliate ID.

I made a custom field called “az-post” which I populate when making the post (or mass importing via CSV). To test things out I added the following code to my post-head.php.

<div class=”single-post-thumb”>
<?php if(get_field(‘az-post’)): ?>
<a href=”http://www.amazon.com/dp/<?php the_field(‘az-post’); ?>?tag=myamazonid-20″><?php tie_thumb( $size ); ?></a>
<?php endif; ?>
</div>

With my affiliate ID entered in the “az-post” custom field…everything works as intended.

The problem is when I have a post with no affiliate id in “az-post”. The featured image does not display in the single post.

How would I get the following logic:

If I make a post that uses my affiliate id via custom fields, then it should append a URL to the featured image with my amazon affiliate id. (like the example code above)

IF a post does NOT have my affiliate ID entered THEN use the original theme code below:

<div class=”single-post-thumb”>
<?php tie_thumb( $size ); ?>
</div>

Thanks!