Simple WP Adjacent Post

simple-wp-adjacent-post.php

<?php

function get_adjacent_post_basic ($direction = 'prev', $date = '') {
    global $wpdb, $post;
    
    if( empty( $date ) )
      $date = $post->post_date;

    $operator = $direction == 'prev' ? '<' : '>';
    $order = $direction == 'prev' ? 'DESC' : 'ASC';

    $p = $wpdb->get_results("
    (
      SELECT
        p.post_date,
        p.post_title,
        p.ID
      FROM
        $wpdb->posts AS p 
      WHERE 
        p.post_date $operator '$date'
        AND p.post_type = 'post'
        AND p.post_status = 'publish'
      ORDER by 
        p.post_date $order
      LIMIT 
        1
    )
    ORDER by post_date ASC ");  

    return $p;
  }

usage.php

<?php

$prev = get_adjacent_post_basic('prev');
$next = get_adjacent_post_basic('next');

Very simple starting point for writing a more complex adjacent post query. For example, here is a query which only returns posts with a specific term from a custom taxonomy:

<?php

function get_adjacent_promoted_posts ($direction = 'prev', $date = '') {
    global $wpdb, $post;
    
    if( empty( $date ) )
      $date = $post->post_date;

    $operator = $direction == 'prev' ? '<' : '>';
    $order = $direction == 'prev' ? 'DESC' : 'ASC';

    $p = $wpdb->get_results("
      SELECT
        p.post_date,
        p.post_title,
        p.ID
      FROM
        $wpdb->posts AS p 
      INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id
      INNER JOIN $wpdb->term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
      WHERE 
        p.post_date $operator '$date'
        AND p.post_type = 'post'
        AND p.post_status = 'publish'
        AND tt.taxonomy = 'rating'
        AND tt.term_id = '1778'
      ORDER by 
        p.post_date $order
      LIMIT 
        1
    ");  

    return $p;
  }