I am working on a system outside WordPress. This system is connected with the same database as my WordPress.

I made a small visual example of the tables below.

wp_posts
+----+----------------+-----------+--------------------+
| ID | post_title     | post_type | guid               |
| 23 | Skateboard     | product   | skateboard-red     |
| 24 | Bike           | product   | bike-white         |
| 56 | skateboard-img | image     | skateboard-red.png |
| 89 | bike-img       | image     | bike-white.png     |
+----+----------------+-----------+--------------------+

wp_postmeta
+---------+---------+----------+------------+
| meta_id | post_id | meta_key | meta_value |
|   145   |    23   |   price  |    22.50   |
|   146   |    23   |   thumb  |      56    |
|   147   |    23   |   stock  |      15    |
|   148   |    24   |   price  |    429.50  |
|   149   |    24   |   thumb  |      89    |
|   150   |    24   |   stock  |      3     |
+---------+---------+----------+------------+

If I want the meta_values from wp_postmeta I can use the JOIN method.

<?php
SELECT wp_posts.ID, wp_posts.post_title, wp_postmeta.meta_value 
FROM wp_posts
JOIN wp_postmeta ON wp_posts.ID = wp_postmeta.post_id
WHERE post_type='product' and post_title='Skateboard'; 

$result     = mysqli_query($connection, $query);
while($row = mysqli_fetch_assoc($result)){
    echo $row['ID'];
    echo $row['post_title'];
    echo $row['meta_value'];
}
?>

Now the data returns

• 23 Skateboard 22.50
• 23 Skateboard 56
• 23 Skateboard 15

How can I get the matched image by this post?

The relation between the post and the image is the wp_postmeta table.