Displaying Posts Using a Custom Select Query

1 Description
- 1.1 Assumptions made in this Article
2 Code for the Page Template
3 Query based on Custom Field and Category
4 Query based on Custom Field and Sorted by Value
5 Query by Category Name and Custom Field Key Sorted by Key's Value
6 Acknowledgements
7 Related
8 Resources

Description

At some point in your WordPress development career you may be presented with the need to display one or more posts using SELECT criteria not provided by WordPress' query_posts architecture. For instance, it may become necessary to JOIN WordPress tables to determine which posts should be displayed, or you may want to use data stored in your own tables to determine which posts should be displayed.

The practical example, outlined below, demonstrates a process of selecting all posts with a particular Custom Field value stored, and displaying them in a Page based on a Page Template. Originally, this code was used to implement a post tagging plugin, which allowed organizing posts in less structured collections than the WordPress Categories. Your own usage may be very different, but the content and example should still give you a useful introduction to the general process involved.

Assumptions made in this Article

Generally, this article assumes you have a working knowledge of PHP, MySQL/MariaDB, and WordPress capabilities.

Specific assumptions for the example, however, are:

You have at least one post with Custom Fields data. The Custom Fields should have a key of 'tag' and a value of 'email'
You have created a Page and associated a Page Template with that page. For this example, assume the Template Name is 'Qbased' and was copied from the wp-content/themes/index.php template. If you are not familiar with this process, follow the instructions in Creating your own Page Templates.
As this is a somewhat advanced developer topic, familiarity with the core WordPress concept of The Loop is suggested.

Code for the Page Template

The query

To begin with, it is necessary to retrieve the recordset containing the posts you want to display. To do this, create a result set using the WordPress $wpdb database class. Note that the MySQL/MariaDB SELECT statement illustrates a simple JOIN. Here, $pageposts will contain an array of objects. Each object will represent a published post that has custom field key-value pair - with the key being 'tag' and the value being 'email':

 <?php

 $querystr = "
    SELECT $wpdb->posts.* 
    FROM $wpdb->posts, $wpdb->postmeta
    WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id 
    AND $wpdb->postmeta.meta_key = 'tag' 
    AND $wpdb->postmeta.meta_value = 'email' 
    AND $wpdb->posts.post_status = 'publish' 
    AND $wpdb->posts.post_type = 'post'
    AND $wpdb->posts.post_date < NOW()
    ORDER BY $wpdb->posts.post_date DESC
 ";

 $pageposts = $wpdb->get_results($querystr, OBJECT);
 
 ?>

The Revised Loop

Now, to display posts collected into $pageposts by the previous SELECT criteria, you need to replace The Loop with your own loop code in the Qbased Page Template. This requires creating a revised loop that cycles through the posts stored in $pageposts and displays them. Note: the structure / markup in the loop below is taken from the WordPress default theme.

 <?php if ($pageposts): ?>
 <?php global $post; ?>
 <?php foreach ($pageposts as $post): ?>
 <?php setup_postdata($post); ?>
 
 <div class="post" id="post-<?php the_ID(); ?>">
 <h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
    <?php the_title(); ?></a></h2>
    <small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
    <div class="entry">
       <?php the_content('Read the rest of this entry »'); ?>
    </div>
    <p class="postmetadata">Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?>  
    <?php comments_popup_link('No Comments »', '1 Comment »', '% Comments »'); ?></p>
 </div>
 <?php endforeach; ?>
 <?php else : ?>
    <h2 class="center">Not Found</h2>
    <p class="center">Sorry, but you are looking for something that isn't here.</p>
    <?php include (TEMPLATEPATH . "/searchform.php"); ?>
 <?php endif; ?>

And that's it!

To go through the important parts of the code, line by line, you have:

A test to make sure that the query that populated $pageposts actually found some posts that matched the SELECT criteria:

<?php if ($pageposts): ?>

A foreach loop to go through the posts returned in $pageposts and display them:

<?php foreach($pageposts as $post): ?>

A declaration that we are using the global WordPress variable $post in order to make the Template Tags work (they will not be populated by setup_postdata() properly otherwise):

<?php global $post; ?>

And, a call to the WordPress post formatting function, setup_postdata(), that automatically populates the required variables:

<?php setup_postdata($post); ?>

Within the Loop

Because setup_postdata($post); was called in our example, you can use the same template tags that can be included in a normal WordPress post loop, like the_content() and the_permalink(). This means that you can create your own post display results using a Page Template with a minimum amount of fuss, automatically taking advantage of the various plugins you may have activated in your WordPress blog to provide extra formatting and functionality.

The Completed Page Template

Here is a complete example of the new template that works with the WordPress default theme.

<?php
/*
Template Name: Qbased
*/
?>

<?php get_header(); ?>

<div id="content" class="narrowcolumn">

<?php

 $querystr = "
    SELECT $wpdb->posts.* 
    FROM $wpdb->posts, $wpdb->postmeta
    WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id 
    AND $wpdb->postmeta.meta_key = 'tag' 
    AND $wpdb->postmeta.meta_value = 'email' 
    AND $wpdb->posts.post_status = 'publish' 
    AND $wpdb->posts.post_type = 'post'
    AND $wpdb->posts.post_date < NOW()
    ORDER BY $wpdb->posts.post_date DESC
 ";

 $pageposts = $wpdb->get_results($querystr, OBJECT);

?>
 <?php if ($pageposts): ?>
  <?php global $post; ?>
  <?php foreach ($pageposts as $post): ?>
    <?php setup_postdata($post); ?>

    <div class="post" id="post-<?php the_ID(); ?>">
      <h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
      <?php the_title(); ?></a></h2>
      <small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
      <div class="entry">
         <?php the_content('Read the rest of this entry »'); ?>
      </div>
  
      <p class="postmetadata">Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?>  
      <?php comments_popup_link('No Comments »', '1 Comment »', '% Comments »'); ?></p>
    </div>
  <?php endforeach; ?>
  
  <?php else : ?>
    <h2 class="center">Not Found</h2>
    <p class="center">Sorry, but you are looking for something that isn't here.</p>
    <?php include (TEMPLATEPATH . "/searchform.php"); ?>
 <?php endif; ?>

</div>

<?php get_sidebar(); ?>

<?php get_footer(); ?>

It is important to note here that the above example will work only when OBJECT or OBJECT_K (for unique results) is passed as the "output_type" parameter for $wpdb->get_results(). setup_postdata() requires an object and will not work when ARRAY_A or ARRAY_N are passed into $wpdb->get_results().

Query based on Custom Field and Category

This next example sets the $querystr variable used in the above example, to get all posts in Categories 1,2, and 3, that have the meta_key 'paragraf', and then sorted ascending by the meta_values. This example gleaned from Otto42's response in Forum Topic 121011.

Use this example for version prior to WordPress 2.3:

 $querystr = "
    SELECT $wpdb->posts.*
    FROM $wpdb->posts
    LEFT JOIN $wpdb->postmeta ON ($wpdb->posts.ID = $wpdb->postmeta.post_id)
    LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id)
    WHERE $wpdb->postmeta.meta_key = 'paragraf'
    AND $wpdb->posts.post_status = 'publish'
    AND $wpdb->posts.post_type = 'post'
    AND $wpdb->post2cat.category_id IN (1,2,3)
    ORDER BY $wpdb->postmeta.meta_value ASC
    ";

Use this example for WordPress Version 2.3: This example gleaned from kernow's response in Forum Topic 121011

SELECT * FROM $wpdb->posts
LEFT JOIN $wpdb->postmeta ON($wpdb->posts.ID = $wpdb->postmeta.post_id)
LEFT JOIN $wpdb->term_relationships ON($wpdb->posts.ID = $wpdb->term_relationships.object_id)
LEFT JOIN $wpdb->term_taxonomy ON($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
WHERE $wpdb->term_taxonomy.term_id IN (1,2,3)
AND $wpdb->term_taxonomy.taxonomy = 'category'
AND $wpdb->posts.post_status = 'publish'
AND $wpdb->postmeta.meta_key = 'paragraf'
ORDER BY $wpdb->postmeta.meta_value ASC

Previous examples don't work with WordPress Version 2.5 or higher.

This example, tested Wordpress 2.9.2, is made to get posts in selected categories that are after a date, sorted ascending. It limits the number to just 4 items returned.

<?php
global $wpdb;
global $post;
$querystr = "
SELECT DISTINCT wposts.* 
FROM $wpdb->posts wposts
	LEFT JOIN $wpdb->postmeta wpostmeta ON wposts.ID = wpostmeta.post_id 
	LEFT JOIN $wpdb->term_relationships ON (wposts.ID = $wpdb->term_relationships.object_id)
	LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
WHERE wpostmeta.meta_key = 'customDateField'
	AND wpostmeta.meta_value >= CURDATE()
	AND $wpdb->term_taxonomy.taxonomy = 'category'
	AND $wpdb->term_taxonomy.term_id IN(1,2)
ORDER BY wpostmeta.meta_value ASC
LIMIT 4
";
?>

The global $wpdb; line stops the Fatal error: Call to a member function get_var() on a non-object errors.

Query based on Custom Field and Sorted by Value

This example pulls pages with a certain custom field key, then sorts on the key's value.

 $querystr = "
    SELECT wposts.* 
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id 
    AND wpostmeta.meta_key = 'custom-key' 
    AND wposts.post_type = 'page' 
    ORDER BY wpostmeta.meta_value DESC
    ";

Query by Category Name and Custom Field Key Sorted by Key's Value

This example selects posts that have a common category name and custom field key, then sorts the selection by the custom key's value.

$querystr = "
	SELECT * FROM $wpdb->posts
	LEFT JOIN $wpdb->postmeta ON($wpdb->posts.ID = $wpdb->postmeta.post_id)
	LEFT JOIN $wpdb->term_relationships ON($wpdb->posts.ID = $wpdb->term_relationships.object_id)
	LEFT JOIN $wpdb->term_taxonomy ON($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
	LEFT JOIN $wpdb->terms ON($wpdb->term_taxonomy.term_id = $wpdb->terms.term_id)
	WHERE $wpdb->terms.name = 'slides'
	AND $wpdb->term_taxonomy.taxonomy = 'category'
	AND $wpdb->posts.post_status = 'publish'
	AND $wpdb->posts.post_type = 'post'
	AND $wpdb->postmeta.meta_key = 'order'
	ORDER BY $wpdb->postmeta.meta_value ASC
";

Acknowledgements

Many thanks to Kafkaesquii for pointing out a simpler method of populating the appropriate global variables, etc, using setup_postdata().

Class: wpdb - Overview of using the $wpdb object
Tutorial: Displaying Posts Using a Custom Select Query
Tutorial: Making Advanced Taxonomy Queries
Action Hook: pre_get_posts - Modify WordPress queries before they are executed
Function: query_posts() - Make additional custom queries
Function: setup_postdata() - Setup query data for individual results within a loop

Resources

Query for multiple post meta values/custom fields

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