Changing regular db connection to $wpdb

Question

I am working on a wordpress site, and started implanting a CRUD database. I coded out an example that works perfectly on my localhost. However, I am working with file mangaer in cpanel, and am having difficulty with changing the regular database query to a one that is incorporated with $wpdb. I have tried to use $wpdb->get_results(), but I get the same error. Any help would be greatly appreciated.

Here is my code:

  // CONNECTION TO DATABASE
    $query = "SELECT * FROM students";
    $query_results = mysqli_query($wpdb->dbh, $query) or die(mysqli_error($wpdb->dbh));

    if(mysqli_num_rows($query_results) > 0) 
    {
        foreach($query_run as $student)
        {
            ?>
            echo '
                <form id="saveStudent">
                    <tr>' 
                        echo '<td>'<?php $student->id; ?> echo '</td>';
                        echo '<td>'<?php $student->first_name; ?> echo '</td>';
                        echo '<td>'<?php $student->last_name; ?> echo '</td>';
                        echo '<button type="button" class="btn btn-danger">Delete Student</button>'

            echo '</tr>';
            
            <?php
        }
    }
    ?>

EDIT: The error message I get is Warning: mysqli_query() expects parameter 1 to be mysqli, null given

Where is this file? Is it someplace inside a theme or plugin where other WordPress functions are working? — WebElaine
– WebElaine, Commented Oct 17, 2022 at 21:10
I am working within the cart page, under the web hook woocommerce_before_cart. The file itself is included in the functions.php file which is apart of cpanel's file manager — John Lyons
– John Lyons, Commented Oct 17, 2022 at 21:14
Have you declared global $wpdb; before trying to use the global $wpdb object? — Pat J
– Pat J, Commented Oct 18, 2022 at 2:08

webaware · Accepted Answer · 2022-10-18 08:22:22Z

0

Use the global $wpdb object to query your tables. The get_results() method will return your results as you need:

global $wpdb;

$query = 'SELECT * FROM students';
$query_results = $wpdb->get_results($query);
foreach ($query_results as $student) {
    // ...
}

answered Oct 18, 2022 at 8:22

webaware

3,3412 gold badges25 silver badges32 bronze badges

When I tried that out, I got Fatal error: Uncaught Error: Call to a member function get_results() on null

John Lyons
– John Lyons

2022-10-18 13:59:44 +00:00
Commented Oct 18, 2022 at 13:59
@JohnLyons check your spelling $wpdb and ensure you are calling this from a WordPress or WooCommerce hook so that the global variable has been defined.

webaware
– webaware

2022-10-18 22:59:08 +00:00
Commented Oct 18, 2022 at 22:59
e.g. add_action('woocommerce_before_cart', function() : void { ... });

webaware
– webaware

2022-10-18 23:00:53 +00:00
Commented Oct 18, 2022 at 23:00

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