In WordPress, taxonomies are a way to organize and group content. They help structure content by classifying posts, pages, and custom post types. WordPress has two built-in taxonomies and allows the creation of custom taxonomies.

1. Built-in Taxonomies

WordPress comes with two default taxonomies:

• Categories (category) – A hierarchical taxonomy used to group posts into broad topics. A post can belong to multiple categories.
• Tags (post_tag) – A non-hierarchical taxonomy that allows freeform labeling of posts. Unlike categories, tags do not have a parent-child structure.

2. Custom Taxonomies

Developers can create custom taxonomies to better organize custom post types. These can be:

• Hierarchical – Like categories, allowing parent-child relationships.
• Non-hierarchical – Like tags, with no parent-child structure.

3. Registering a Custom Taxonomy

Custom taxonomies are registered using register_taxonomy(). Example:

function custom_taxonomy() {
    register_taxonomy(
        'genre', // Taxonomy slug
        'book', // Post type
        array(
            'label' => __('Genre'),
            'rewrite' => array('slug' => 'genre'),
            'hierarchical' => true, // True for categories, false for tags
        )
    );
}
add_action('init', 'custom_taxonomy');

This code creates a “Genre” taxonomy for a custom post type “Book”.

4. Displaying Taxonomies

To display taxonomy terms in a template:

$terms = get_the_terms(get_the_ID(), 'genre');
if ($terms && !is_wp_error($terms)) {
    foreach ($terms as $term) {
        echo $term->name . ' ';
    }
}

5. Querying Posts by Taxonomy

You can retrieve posts based on taxonomy using WP_Query:

$args = array(
    'post_type' => 'book',
    'tax_query' => array(
        array(
            'taxonomy' => 'genre',
            'field'    => 'slug',
            'terms'    => 'fiction',
        ),
    ),
);
$query = new WP_Query($args);

Database structure

In WordPress, taxonomies and their terms are stored in multiple database tables. Here’s how they are structured:

1. wp_terms (Stores Term Names)

This table holds the actual terms (categories, tags, or custom taxonomy terms).

• term_id → Unique ID for the term.
• name → The term’s human-readable name.
• slug → URL-friendly version of the term.
• term_group → Used for grouping terms (rarely used).

2. wp_term_taxonomy (Links Terms to Taxonomies)

This table defines which taxonomy a term belongs to.

• term_taxonomy_id → Unique ID linking terms to taxonomies.
• term_id → References wp_terms.term_id.
• taxonomy → Name of the taxonomy (e.g., category, post_tag, or custom like genre).
• description → Description of the term.
• parent → Parent term ID (for hierarchical taxonomies).
• count → Number of posts associated with this term.

3. wp_term_relationships (Links Terms to Posts)

This table connects taxonomy terms to posts (or custom post types).

• object_id → The ID of the post (from wp_posts.ID).
• term_taxonomy_id → Links to wp_term_taxonomy.term_taxonomy_id.
• term_order → Controls order (not commonly used).

4. wp_termmeta (Stores Term Metadata)

Introduced in WordPress 4.4, this table stores custom metadata for terms.

• meta_id → Unique ID for the metadata entry.
• term_id → References wp_terms.term_id.
• meta_key → Name of the metadata field.
• meta_value → Value of the metadata field.

How Taxonomy Queries Work

When WordPress queries posts by taxonomy, it joins these tables:

1. Find the term ID in wp_terms.
2. Get the taxonomy from wp_term_taxonomy using term_id.
3. Retrieve posts from wp_term_relationships using term_taxonomy_id.

Example SQL query:

SELECT p.* FROM wp_posts p
JOIN wp_term_relationships tr ON p.ID = tr.object_id
JOIN wp_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
JOIN wp_terms t ON tt.term_id = t.term_id
WHERE tt.taxonomy = 'genre' AND t.slug = 'fiction';