get_adjacent_posts()

gistfile1.php

<?php 
/**
 * Retrieve adjacent post.
 *
 * Can either be next or previous post.
 *
 * @since 2.5.0
 *
 * @param bool         $in_same_term   Optional. Whether post should be in a same taxonomy term.
 * @param array|string $excluded_terms Optional. Array or comma-separated list of excluded term IDs.
 * @param bool         $previous       Optional. Whether to retrieve previous post.
 * @param string       $taxonomy       Optional. Taxonomy, if $in_same_term is true. Default 'category'.
 * @return mixed       Post object if successful. Null if global $post is not set. Empty string if no corresponding post exists.
 */
function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
    global $wpdb;

    if ( ( ! $post = get_post() ) || ! taxonomy_exists( $taxonomy ) )
        return null;

    $current_post_date = $post->post_date;

    $join = '';
    $posts_in_ex_terms_sql = '';
    if ( $in_same_term || ! empty( $excluded_terms ) ) {
        $join = " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";

        if ( $in_same_term ) {
            if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) )
                return '';
            $term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );
            if ( ! $term_array || is_wp_error( $term_array ) )
                return '';
            $join .= $wpdb->prepare( " AND tt.taxonomy = %s AND tt.term_id IN (" . implode( ',', array_map( 'intval', $term_array ) ) . ")", $taxonomy );
        }

        $posts_in_ex_terms_sql = $wpdb->prepare( "AND tt.taxonomy = %s", $taxonomy );
        if ( ! empty( $excluded_terms ) ) {
            if ( ! is_array( $excluded_terms ) ) {
                // back-compat, $excluded_terms used to be $excluded_terms with IDs separated by " and "
                if ( false !== strpos( $excluded_terms, ' and ' ) ) {
                    _deprecated_argument( __FUNCTION__, '3.3', sprintf( __( 'Use commas instead of %s to separate excluded terms.' ), "'and'" ) );
                    $excluded_terms = explode( ' and ', $excluded_terms );
                } else {
                    $excluded_terms = explode( ',', $excluded_terms );
                }
            }

            $excluded_terms = array_map( 'intval', $excluded_terms );

            if ( ! empty( $term_array ) ) {
                $excluded_terms = array_diff( $excluded_terms, $term_array );
                $posts_in_ex_terms_sql = '';
            }

            if ( ! empty( $excluded_terms ) ) {
                $posts_in_ex_terms_sql = $wpdb->prepare( " AND tt.taxonomy = %s AND tt.term_id NOT IN (" . implode( $excluded_terms, ',' ) . ')', $taxonomy );
            }
        }
    }

    $adjacent = $previous ? 'previous' : 'next';
    $op = $previous ? '<' : '>';
    $order = $previous ? 'DESC' : 'ASC';

    /**
     * Filter the JOIN clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, $adjacent, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     *
     * @param string $join           The JOIN clause in the SQL.
     * @param bool   $in_same_term   Whether post should be in a same taxonomy term.
     * @param array  $excluded_terms Array of excluded term IDs.
     */
    $join  = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms );

    /**
     * Filter the WHERE clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, $adjacent, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     *
     * @param string $where          The WHERE clause in the SQL.
     * @param bool   $in_same_term   Whether post should be in a same taxonomy term.
     * @param array  $excluded_terms Array of excluded term IDs.
     */
    $where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s AND p.post_status = 'publish' $posts_in_ex_terms_sql", $current_post_date, $post->post_type), $in_same_term, $excluded_terms );

    /**
     * Filter the ORDER BY clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, $adjacent, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     *
     * @param string $order_by The ORDER BY clause in the SQL.
     */
    $sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

    $query = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
    $query_key = 'adjacent_post_' . md5( $query );
    $result = wp_cache_get( $query_key, 'counts' );
    if ( false !== $result ) {
        if ( $result )
            $result = get_post( $result );
        return $result;
    }

    $result = $wpdb->get_var( $query );
    if ( null === $result )
        $result = '';

    wp_cache_set( $query_key, $result, 'counts' );

    if ( $result )
        $result = get_post( $result );

    return $result;
}
?>