How do I query a single data value from the wp_metadata table?

You need to use get_post_meta:

get_post_meta( $post_id, $key = ”, $single = false )

Retrieve post meta field for a post

$post_id (int) (Required) Post ID.

$key (string) (Optional) The meta key to retrieve. By default, returns data for all keys. Default value: ”

$single (bool) (Optional) Whether to return a single value. Default value: false

eg.

echo get_post_meta( $post->ID, 'the-name-of-key', true); 

When using true, you will get a string, but if you use false instead, you will get an array.