How to create WordPress Related Posts | Article on www.ibenic.com

content.php

<?php

add_filter( 'the_content','ibenic_related_content' );
function ibenic_related_content( $content ){
  
  // Check if the content is of a single article
  if( is_singular( 'post' ) ) {
    global $post;
    // Variable that will be used to create the HTML of related content
    $related_content = "";
    
    $articles = ibenic_get_related_content( $post->ID );
    
    if( ! empty( $articles ) ){
      
      $related_content .= "<h2>" . __( "Related Content","your_textdomain" ) . "</h2>";
      $related_content .= "<ul>";
      
      foreach($articles as $article){
      
        $related_content .= "<li><a href='" . get_permalink( $article->ID ) . "'>";
        $related_content .=  "<h3>" . $article->post_title . "</h3>";
        // You can set here other $article information such as a thumbnail, category, date etc.
        $related_content .= "</a></li>";
        
      }
      
      $related_content .= "</ul>";
      $content .= $related_content;
      return $content;
    }
    
  } else {
    return $content;
  }
}

function_query.php

<?php
// ....
$sql = "SELECT * FROM $wpdb->posts WHERE ID IN (
         SELECT post_id FROM (
          SELECT t1.object_id as post_id, count(t1.term_taxonomy_id) as term_match
          FROM $wpdb->term_relationships as t1
          INNER JOIN $wpdb->term_relationships as t2 on t1.term_taxonomy_id = t2.term_taxonomy_id
          
          -- Joining the table that describes the taxonomies
          INNER JOIN $wpdb->term_taxonomy as x1 on x1.term_taxonomy_id = t2.term_taxonomy_id
          
          -- Filtering the query with another WHERE condition where we want only terms of taxonomy with the name 'tag'
          WHERE t2.object_id = $ID AND x1.taxonomy = 'tag'
          
          GROUP BY t1.object_id
          HAVING term_match > 4 AND t1.object_id != $ID 
          LIMIT 20
         ) as related_content
       )";
 //...

function_query2.php

<?php

$sql = "SELECT * FROM $wpdb->posts WHERE ID IN (
         SELECT post_id FROM (
          SELECT t1.object_id as post_id, count(t1.term_taxonomy_id) as term_match
          FROM $wpdb->term_relationships as t1
          INNER JOIN $wpdb->term_relationships as t2 on t1.term_taxonomy_id = t2.term_taxonomy_id
          -- Joining the table that describes the taxonomies
          INNER JOIN $wpdb->term_taxonomy as x1 on x1.term_taxonomy_id = t2.term_taxonomy_id
          
           -- Joining the table that that contains meta data to each article by the ID of the article 
           -- and the post_id of the meta data table
          INNER JOIN $wpdb->postmeta as m1 on m1.post_id = t1.object_id
          
          
          -- Filtering the query with another WHERE condition where we want only terms of taxonomy with the name 'tag'
          WHERE t2.object_id = $ID AND x1.taxonomy = 'tag'

          -- Filtering the query with yet other WHERE conditions where we want to limit the query to a specific meta data
          AND m1.meta_key = 'META_KEY_WE_WANT_TO_LOOK_FOR'
          AND m1.meta_value = 'META_VALUE_of_the_META_KEY_WE_WANT_TO_LOOK_FOR'
          
          GROUP BY t1.object_id
          HAVING term_match > 4 AND t1.object_id != $ID 
          LIMIT 20
         ) as related_content
       )";

main_and_sub_query.php

<?php
//Main Query
SELECT * FROM wp_posts WHERE ID IN(

  //Subqery
  SELECT post_id FROM ( 
  
    /*Get IDs of articles and the number of same terms with the main article*/
    
    SELECT t1.object_id as post_id, count(t1.term_taxonomy_id) as term_match
    FROM wp_term_relationships as t1
    
    /*Join the table that represents the list of all articles with the table of the main article*/
    
    INNER JOIN wp_term_relationships as t2 on t1.term_taxonomy_id = t2.term_taxonomy_id
    
    /*Filter the query by limit the t2 table only to the main article*/
    
    WHERE t2.object_id = MAIN_ARTICLE_ID
    
    /*Group by IDs of the table t1 (all articles) so that we do not get rows with duplicate IDs*/
    
    GROUP BY t1.object_id
    
    /* Give a condition to the query so that the query returs only articles that have at least 5 of terms that are common
    to the main article. We are also excluding the ID of our main article from the list of all articles
    */
    
    HAVING term_match > 4 AND t1.object_id != MAIN_ARTICLE_ID
    
    /*Limit the result to 20 rows */
    
    LIMIT 20
  ) as related_content
)

query.php

<?php

/*Get IDs of articles and the number of same terms with the main article*/

SELECT t1.object_id as post_id, count(t1.term_taxonomy_id) as term_match
FROM wp_term_relationships as t1

/*Join the table that represents the list of all articles with the table of the main article*/

INNER JOIN wp_term_relationships as t2 on t1.term_taxonomy_id = t2.term_taxonomy_id

/*Filter the query by limit the t2 table only to the main article*/

WHERE t2.object_id = MAIN_ARTICLE_ID

/*Group by IDs of the table t1 (all articles) so that we do not get rows with duplicate IDs*/

GROUP BY t1.object_id

/* Give a condition to the query so that the query returs only articles that have at least 5 of terms that are common
to the main article. We are also excluding the ID of our main article from the list of all articles
*/

HAVING term_match > 4 AND t1.object_id != MAIN_ARTICLE_ID

/*Limit the result to 20 rows */

LIMIT 20

related_posts.php

<?php

function ibenic_get_related_content($ID){
  global $wpdb;

 //Here goes the code

}

related_posts2.php

<?php

function ibenic_get_related_content($ID){
  global $wpdb;

  // Here goes the code

  // First, we check if the parameter $ID is a number. If not, the function returns false
  if( ! is_numeric( $ID ) ){
    return false;
  }
  
  // We are creating an integer out of it
  $ID = intval( $ID );
  
  /*
  * If the integer is 0, then the $ID was not successfully converted 
  * and there is no article with the ID equal to zero
  */
  if( $ID === 0 ){
   return false;
  }
  
  // $ID is ok and we can continue performing the query
  $sql  = "SELECT * FROM $wpdb->posts WHERE ID IN (
            SELECT post_id FROM (
              SELECT t1.object_id as post_id, count(t1.term_taxonomy_id) as term_match
              FROM $wpdb->term_relationships as t1
              INNER JOIN $wpdb->term_relationships as t2 on t1.term_taxonomy_id = t2.term_taxonomy_id
              WHERE t2.object_id = $ID 
              GROUP BY t1.object_id
              HAVING term_match > 4 AND t1.object_id != $ID 
              LIMIT 20
            ) as related_content
         )";
  // Return an array with objects of each post
  return $wpdb->get_results( $sql );

}