use mysql to replace one custom field value with another custom field value

I’m not sure you can do this with MySQL alone, since you’d need to save values and reuse them. Here’s a PHP version.

<?php
global $wpdb;
// select all the old postmeta
$oldData = $wpdb->get_results("SELECT * FROM wp_postmeta WHERE meta_key = 'tnid_01old'");
// make sure we have results
if($oldData) {
    // loop through each result
    foreach($oldData as $meta) {
        // save post ID and meta value
        $meta_post_id = $meta->post_id;
        $metavalue = $meta->meta_value;
        // update 'tnid_01' value
        $wpdb->update(
            // table name
            'wp_postmeta',
            // value to set
            array(
                'meta_value' => "$metavalue"
            ),
            // condition to meet: match same post ID but new meta key
            array(
                'post_id' => $meta_post_id,
                'meta_key' => 'tnid_01'
            )
        )
    }
}
?>

If you’re on a low-traffic site, you could temporarily paste this into a theme template file and load whatever page uses that template, then make sure to remove this code so it’s not hitting the database again. Or a more proper way would be to have it run within wp-admin.

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