Why $wpdb->show_errors() and print_error() is showing an output even if the query output is correct?

Question

In order of figuring out the following issue, see https://wordpress.stackexchange.com/questions/178995/sanitize-a-working-query-string-by-using-wpdb-prepare-fails-with-mysql-db-er i ran into an rather odd behaviour. Even that my used query was correct and showing the right output.

global $wpdb;
$wpdb->show_errors(); 
$pageposts = $wpdb->get_results( 
        $wpdb->prepare(
          "
          SELECT skposts.*
          FROM  $wpdb->posts skposts,
                $wpdb->postmeta skpostmeta1,
                $wpdb->postmeta skpostmeta2
          WHERE skposts.ID = skpostmeta1.post_id
            AND skposts.ID = skpostmeta2.post_id
            AND skpostmeta1.meta_key = %s
            AND skpostmeta2.meta_key = %s
            AND skposts.post_type = %s
            AND skposts.post_status = %s
          ORDER BY skpostmeta1.meta_value ASC,
                   skpostmeta2.meta_value ASC
          ", 
          'class_day',
          'class_start',
          'courses',
          'publish'
        )
, OBJECT
);
$wpdb->print_error();

Never the less as long as i have show_errors and print_error active i get an WP database error output alongside:

WordPress database error: []
SELECT skposts.* FROM hmjtZ_posts skposts, hmjtZ_postmeta skpostmeta1, hmjtZ_postmeta skpostmeta2 WHERE skposts.ID = skpostmeta1.post_id AND skposts.ID = skpostmeta2.post_id AND skpostmeta1.meta_key = 'class_day' AND skpostmeta2.meta_key = 'class_start' AND skposts.post_type = 'courses' AND skposts.post_status = 'publish' ORDER BY skpostmeta1.meta_value ASC, skpostmeta2.meta_value ASC

But why? I would have expected that an output is shown only if something goes wrong like an error or warning but this way something is shown all the time.

Answer 1

The output that you posted above is expected behaviour for $wpdb->print_error() if the following is true -

• You are running a single site, not multisite
• $wpdb->suppress_errors is set to false
• $wpdb->show_errors is set to false

From the looks of your code, you meet all those conditions.

Note also that, unless you have turned them off previously, $wpdb->show_errors is set to true by default, so you don't need to call $wpdb->show_errors().

To output something only when there is a DB error you can do one of these two things -

1 - Output the error and add the error to the log

As well as outputting on the screen, the $wpdb->print_error() method will log your error. If this is desirable behaviour (recommended), you can do this -

if($wpdb->last_error !== '') :
    $wpdb->print_error();
endif;

2 - Output the error but do not log it

If you are not interested in logging the error, you can add your own my_print_error() funciton and use that instead of $wpdb->print_error() -

function my_print_error(){

    global $wpdb;

    if($wpdb->last_error !== '') :

        $str   = htmlspecialchars( $wpdb->last_result, ENT_QUOTES );
        $query = htmlspecialchars( $wpdb->last_query, ENT_QUOTES );

        print "<div id='error'>
        <p class='wpdberror'><strong>WordPress database error:</strong> [$str]<br />
        <code>$query</code></p>
        </div>";

    endif;

}

Last Edit: Syntax Mistake