plugin code is pulling information from database in one instance and not in other. What is wrong?

Question

In my plugin code, I am pulling the table id from two tables. The id is being pulled from one table but not from the other and I get errors in both instances. The queries work on mysql when I use them. any thoughts? code is below and output below that. The id gets pulled from the wp_shclass table but not the wp_shinstructor table. Any thoughts I have been staring at this for a few days and am not sure what's wrong. I am using wp 3.3.1

function shmain_admin(){
if (!current_user_can('manage_options'))  {
wp_die( __('You do not have sufficient permissions to access this page.') );
}

echo "Hello class";
global $wpdb;
$shrow = $wpdb->get_row("select * from wp_shclass ORDER BY classid DESC LIMIT 1", ARRAY_A);  
$shclassid = $shrow['classid'];
echo "<br> this is " . $shclassid;
$wpdb->show_errors();
$wpdb->print_error();
echo "<br> Hello instructor";
$shrow = $wpdb->get_row("select * from wp_shinstructor ORDER BY instructorid DESC LIMIT 1", ARRAY_A);  
$shinstructorid = $shrow['instructorid'];
echo "<br> this is " . $shinstructorid;
$wpdb->print_error();
}

function shmain_admin_action(){
add_menu_page('sh main', 'main', 'manage_options','shmain', 'shmain_admin');
}

add_action('admin_menu','shmain_admin_action');

and here is the output below

---

Hello class this is 8

WordPress database error: []
select * from wp_shclass ORDER BY classid DESC LIMIT 1

Hello instructor this is

WordPress database error: []
select * from wp_shinstructor ORDER BY instructorid DESC LIMIT 1

Answer 1

In general, I'd recommend to read about $wpdb in detail in the Codex.

Some notes:

1. Never use the default wp_ prefix.
2. Never set a hard coded prefix. It will be set during installation and should later be added using $wpdb->prefix. But: Prefixes can be changed with the following SQL command. "RENAME table wp_posts TO {$wpdb->prefix}posts;"
3. Normally there's always a way to get around custom tables. CPTs and Custom Taxonomies are just an example.

Answer 2

Deleting the tables and recreating them solved the problem. The problem was in how I had created the tables