Query to display post by category using Joins.
<?php
global $wpdb, $post;
$slug = get_post( $post )->post_name;
$query = "SELECT * FROM wp_posts p LEFT OUTER JOIN wp_term_relationships r ON r.object_id = p.ID LEFT OUTER JOIN wp_term_taxonomy x ON x.term_taxonomy_id = r.term_taxonomy_id LEFT OUTER JOIN wp_terms t ON t.term_id = x.term_id WHERE p.post_status = 'publish' AND p.post_type = 'post' AND t.slug = '$slug' order by ID Desc limit 8";
?>
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