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How can I spit out a list of values of a custom field, suppressing duplicate values?

More specifically...

For custom post named "pressmention", I want to output a file, screen echo or web page of values in its custom text field named "press_mention_information_publication-name".

However, the output should only spit out a value once.

That is to say, "pressmention" has approximately 70,000 posts. For a quantity of those (let's say, 1,000), "press_mention_information_publication-name" may have value "Wall Street Journal". But I would only want to see "Wall Street Journal" output once.

This is not for a front-end feature, it does not need to exist on WordPress. It is for my information.

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Here's a dashboard widget you could manage them with:

// add dashboard widget
add_action('wp_dashboard_setup', 'wpse_dashboard_widget');
function wpse_dashboard_widget() {
    // only show widget to users who can publish pages
    if(current_user_can('publish_pages')) {
        wp_add_dashboard_widget('deduped_dash_widget', 'Publications', 'wpse_create_deduped_dash_widget');
    }
}
// widget contents
function wpse_create_deduped_dash_widget() {
    // select publications without duplicates by using DISTINCT
    global $wpdb;
    $mentions = $wpdb->get_results("SELECT DISTINCT meta_value FROM wp_postmeta WHERE meta_key = 'press_mention_information_publication-name'");
    // if any were found
    if($mentions) {
        // display them in an unordered list; could do <ol> if you want numbers
        echo '<ul>';
        foreach($mentions as $publication) {
            echo '<li>' . $publication->meta_value) . '</li>';
        }
        echo '</ul>';
    }
}

If you have a custom theme or custom child theme you can place this in functions.php. Otherwise, you can create a plugin which may be easier than creating a child theme just for this widget.

Or, if you have access to phpMyAdmin, you could run the query

SELECT DISTINCT meta_value FROM wp_postmeta WHERE meta_key = 'press_mention_information_publication-name'

and get the list as well.

  • The SQL works, but the dashboard widget throws an error, here it is via debug logging... Parse error: syntax error, unexpected ')', expecting ',' or ';' in /home/sitename/public_html/wp-content/themes/blankslate/functions.php on line 277 Yes, I hve a plugin in iamp-index-management. – Robert Andrews May 26 '17 at 21:20
  • I have fixed part of the error by removing the rogue close-bracket. Now I get to see the dashboard. But this produces a few more error messages... - one in debug loggin Notice: Use of undefined constant wpse_create_deduped_dash_widget - assumed 'wpse_create_deduped_dash_widget' in /home/braincloud/public_html/wp-content/themes/blankslate/functions.php on line 265 and two within the dashboard widget: Notice: Undefined variable: wpdb in /home/sitename/public_html/wp-content/themes/blankslate/functions.php on line 271 – Robert Andrews May 26 '17 at 21:26
  • Fatal error: Call to a member function get_results() on a non-object in /home/sitename/public_html/wp-content/themes/blankslate/functions.php on line 271 Sorry I haven't figured out how to line-break in a comment. – Robert Andrews May 26 '17 at 21:26
  • 1) I fixed the error message about wpse_create_deduped_dash_widget by putting single quotes around both ends. 2) But there was still an error about an undefined $wpdb. I added global $wpdb;, can you confirm that's right? 3) That left an error from SQL output within your dashboard widget box concerning not finding the post_meta. So I tweaked the SQL query. 4) I have working code at a fiddle URL, though not sure rules permit me to link. – Robert Andrews May 26 '17 at 21:45
  • @Robert: I've updated the answer - yes, a quote was missing and you do need global $wpdb. You should be able to link to a fiddle URL here and I'm happy to update the answer with the updates, sorry it didn't work as-is but to hear glad you got it working. – WebElaine May 30 '17 at 14:16

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