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I am using $wpdb->update to update a custom table I have in my database. When I var_dump the result it returns:

int(0)

So I tried $wpdb->print_error() to see what's wrong. However, it shows nothing. I also tried $wpdb->show_errors(), but again it showed nothing.

The wpdb docs don't go into much detail on how to use these functions, so I am unsure if I am using them correctly. But why would the result of updating a table return 0, and not show any errors?

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5 Answers 5

up vote 9 down vote accepted

I would recommend running the following code right after your query to see what's happening:

exit( var_dump( $wpdb->last_query ) );

This should print the last query that hit your database. In cases like these, I usually will manually run such a query through phpMyAdmin to see if it runs without errors and to see if it even affects the database. Additionally, by seeing the query that was actually run, you may find problems in the query that results from your code. For instance, the query may not return any MySQL errors, but it could run a query that is different than you expect it to be. With this debug code, you'll at least be able to see what it is and continue on the wonderful debug trail! Furthermore, you may want to explore more of the "Class Variables" (Codex Ref) for $wpdb as they may help further troubleshoot your issue.

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Wow been looking for this –  Carl Lindberg Mar 19 at 13:36

Show errors:

  • $wpdb->show_errors = true shows errors automatically, if WP_DEBUG is set to true.
  • $wpdb->suppress_errors = false stops suppressing errors.

  • Multisite need special treatment

    // Show errors in Multisite:
    global $wpdb, $blog_id;
    // There's no is_multisite(), so we need to check the ID
    // This means, that we can't debug the blog with the ID 1 as MU-blog by default
    // Check if we are on Blog ID#1 and if not, check the defines and add error handling
    if ( 1 !== $blog_id )
        ! defined( 'DIEONDBERROR' ) AND define( 'DIEONDBERROR', true );
    

Output handling

The $wpdb->update() method has three different outputs. To check against it, you must save the result i a var: $result = $wpdb->update( /* ... */ );.

The handle those scenarios:

  • false === $result: Fail
  • 0 === $result: Success, but no update
  • 0 < $result: Success

Class output

  • $wpdb->last_error will show you the last error, if you got one.
  • $wpdb->last_query will assist you with showing the last query (where the error occurred). It is basically the same as array_pop( $wpbd->queries );.

Important (security) Note

Please do NOT add this codes on live site. Especially not if caching plugins are involved. This may expose important DB-related data to visitors!

If you can't do otherwise: always wrap your code in conditional statements to prevent public facing debug output!

// Example
function debug_query( $result, $data )
{
    global $current_user;
    get_currentuserinfo();

    if ( current_user_can( 'manage_options' ) )
    {
        global $wpdb, $blog_id;
        1 !== $blog_id
            AND ! defined( 'DIEONDBERROR' )
                AND define( 'DIEONDBERROR', true );

        $wpdb->show_errors     = true;
        $wpdb->suppress_errors = false;

        $output = '<pre style="white-space:pre-line;">';
            $output .= 'Last Error: ';
            $output .= var_export( $wpdb->last_error, true );

            $output .= "\n\nLast Query: ";
            $output .= var_export( $wpdb->last_query, true );

            if ( false === $result )
            {
                $result = new WP_Error( 'query_failed', 'No update.', $data );
            }
            elseif ( 0 === $result )
            {
                $result = new WP_Error( 'update_failed', 'Updated zero rows.', $data );
            }
            elseif ( 0 < $result )
            {
                $result = 'Success';
            }
        $output .= '</pre>';

        // Only abort, if we got an error
        is_wp_error( $result ) 
            AND exit( $output.$result->get_error_message() );
    }
}

Exposing the $wpdb object might also expose your database username and password!

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A zero response means zero rows affected, which is different from an error.

Its hard to say without looking at your query why no rows are being updated. One debug tool you can try is setting "SAVEQUERIES" to true in your wp-config.php file.

Then after your query runs, try var_dumping $wpdb->queries.

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$wpdb->show_errors();
$wpdb->print_error();
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Try this before your query:

$wpdb->show_errors = TRUE;
$wpdb->suppress_errors = FALSE;

or perhaps this after your query:

if ($wpdb->last_error) {
  die('error=' . var_dump($wpdb->last_query) . ',' . var_dump($wpdb->error));
}
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