Count post meta by post_id

post-meta-count.php

<?php

function wpdc_count_post_meta( $post_id  ){

  global $wpdb;

  $sql = $wpdb->prepare( 'SELECT COUNT( meta_id ) FROM '.$wpdb->postmeta.' WHERE `post_id` = %d', $post_id );

  $post_meta_count = $wpdb->get_var( $sql );

  return $post_meta_count;

}

A couple of things for you:

1. For the FROM, just do {$wpdb->postmeta} as this is already a property the WPDB class.
2. What about if that post_id does not exist? What are you wanting to return from this function?
3. If you opt to just return from get_var, then there is no need to assign it to a variable first before returning.

1. Good point. I wasn't aware there were properties for core tables. Makes sense.
   I'm not a fan of the {var} style and prefer to make concatenation clearer.
2. It would return 0 from MySQL. Could I return a WP_Error or false? Sure, but 0 is also true ;-)
3. Yeah, that'd be shorter but assigning it to a variable makes the code read easier (what are we expecting back -- var makes that clearer) and makes debugging easier.

1. Concatenation works too.
2. Since we are doing COUNT() it will return 0 if the post_id does not exist. Other SQL queries using get_var() would return NULL if nothing was found. I just wanted you to think about it a bit. Good job.
3. For optimization, assigning to a variable when that variable is not be reused does take up memory and CPU execution time, albeit small. I just wanted you to think about the efficiency side of coding.

Excellent job! :)