错误执行代码时错误日志中显示错误的SQL查询

Question

I have been going though the WordPress Codex, and it seems as though my syntax is correct, but I can't seem to track down why I keep getting line after line of errors in my errors.text file related to the code below, which is for a WordPress Shortcode:

function blahblah_display_referrer() {
    global $wpdb, $user_ID;

    // Logged in user
    if ( is_user_logged_in() == true ) {
        $sql = "SELECT display_name FROM " .$wpdb->prefix. "users WHERE ID=".$user_ID;
        $ref = $wpdb->get_var($wpdb->prepare($sql));
        return 'Welcome Back: '.$ref;
    }

    // Visitor message with cookie or without...
    $ref = $_COOKIE['ref'];         
    $sql = "SELECT display_name FROM " .$wpdb->prefix. "users WHERE ID=".$ref;
    $ref = $wpdb->get_var($wpdb->prepare($sql));
    if ( !isset($ref) ) {
        return 'Welcome Visitor';
    }

    return 'Referred By: '.$ref;
}

As I said before, this code executes perfectly without any issue. It just shows the following error:

[10-Jul-2012 15:10:45] WordPress database error You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server version for the right syntax to 
use near '' at line 1 for query SELECT display_name FROM wp_users WHERE ID= made by 
require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), 
include('/themes/kaboodle/index.php'), get_sidebar, locate_template, load_template, 
require_once('/themes/kaboodle/sidebar.php'), woo_sidebar, dynamic_sidebar, 
call_user_func_array, WP_Widget->display_callback, WP_Widget_Text->widget, 
apply_filters('widget_text'), call_user_func_array, do_shortcode, preg_replace_callback, 
do_shortcode_tag, call_user_func, blahblah_display_referrer

Here is my server information:

Apache version  2.2.21
PHP version     5.2.17
MySQL version   5.1.63-cll
Architecture    x86_64
Operating system    linux

Answer 1

It would seem that Wordpress's WPDB database module suppresses SQL errors by default .

You can turn error echoing on and off with the show_errors and hide_errors, respectively.

 <?php $wpdb->show_errors(); ?> <?php $wpdb->hide_errors(); ?> 

You can also print the error (if any) generated by the most recent query with print_error.

 <?php $wpdb->print_error(); ?> 

Could that be the problem?

Answer 2

I've had this same issue, and the answer is in the error, but it's not obvious. You're running the Ref query every time someone comes to the page regardless of whether they have a cookie or not. When you're testing, you're most likely testing with a cookie so it's not generating the error. However, when it runs without a cookie, it's searching for a blank ID.

I've modified your code below and commented the changes.

// Visitor message with cookie or without...
$ref = $_COOKIE['ref'];         
  /* This section should only run if there is a ref from the cookie
    $sql = "SELECT display_name FROM " .$wpdb->prefix. "users WHERE ID=".$ref;
    $ref = $wpdb->get_var($wpdb->prepare($sql)); */
if ( !isset($ref) || $ref == "" ) { //In case the cookie returns blank instead of null
    return 'Welcome Visitor';
} else { //added this section after the check for $ref so it only runs where there is a ref
    $sql = "SELECT display_name FROM " .$wpdb->prefix. "users WHERE ID=".$ref;
    $ref = $wpdb->get_var($wpdb->prepare($sql));
}    
return 'Referred By: '.$ref;

Hope this helps.