Inserting Post Meta From SQL

Question

I'm trying to get post meta to be updated/replaced with $wpdb and running into issues. I have working SQL, but cannot get it work when coding it up for my WP plugin.

Extra note:

The post meta being saved is for example is Twitter follower count that I need the number to be saved to the database so I can export the result in a table

Here is my working SQL. When I put this in PHPmyAdmin it updates all post meta:

INSERT INTO wp_postmeta (post_id, meta_key, meta_value)
SELECT wp_posts.ID, 'agency_details_seo_grade', 'test'
FROM wp_posts
WHERE wp_posts.post_status = 'publish' and wp_posts.post_type = 'agencies'";

Here is my failed attempt putting it in WP. Nothing happened as a result (no meta changed and no error message):

// Failed attempt to put into functions.php
function add_grading_post_meta() {
    global $wpdb;
    $wpdb->prepare("SELECT ID,'','test2'", $wpdb->query("INSERT INTO wp_postmeta ($post_id,meta_key,meta_value)"));
}

I'm trying to get this working so I can use it with wp_cron before the next step of using it with WP scheduling functions. Any help would be appreciated for this first step, cracking a lot of hours on this project learning PHP.

Edit:

Here's 1 more version I tried with no success. I changed it due to @czerspalace comment, coding provided by old Stack Exchange post.

function add_grading_post_meta() {
    global $wpdb;

    $twitter_followers = 'agency_details_twitter_followers';
    $twitter_count = 'test3';

    $sql = "INSERT INTO $wpdb->wp_postmeta (post_id,meta_key,meta_value) VALUES   (%d,'%s',%s) ON DUPLICATE KEY UPDATE meta_value = %s";
    $sql = $wpdb->prepare($sql,$post_id,$twitter_followers,$twitter_count);
    $wpdb->query($sql);
}

Answer 1

Your use of $wpdb->prepare is very broken. I honestly don't even know exactly what query you are trying get. The code is so broken I can't even infer what you are trying to do. However, I am guessing you want something like this:

$post_id = 1;
$key = 'abc';
$value = 'def';
$qry = "INSERT INTO wp_postmeta (post_id,meta_key,meta_value) VALUES (%d,%s,%s)";
$qry = $wpdb->prepare(
  $qry,
  $post_id,
  $key,
  $value
);
var_dump($qry);
$wpdb->query($qry);

Notice what happens. You create a "format" string with placeholders for you dynamic data. Then you pass that through prepare along with the replacement strings for the placeholders, and get back a string that is sent through query.

Answer 2

Actually, the customary way to do this in WordPress would be to use <?php update_post_meta($post_id, $meta_key, $meta_value, $prev_value); ?> as per the codex: https://codex.wordpress.org/Function_Reference/update_post_meta. It yields shorter code, and the intent of the code is clearer.