How to Delete a Whole Taxonomy from the Database

To carry out such a task, there is no utility function made up on purpose in WordPress. After all, deleting all the data associated to a taxonomy isn't a common task, or at least not something you routinely do. Me, for example, I ran into the problem while writing the uninstall.php script for SiteTree Pro 4.0 — a file loaded when not all the WordPress functions are available, either. But even considering a more generic context, the most streamlined solution that can be adopted is to use three custom queries:

Copy1global $wpdb;
2
3$taxonomy_name = 'the_taxonomy_to_delete';
4
5# Deletes all the terms associated to the taxonomy.
6#
7# The inner query collects from the 'term_taxonomy' 
8# table all the ids of the terms to delete.
9$wpdb->query( 
10  "DELETE FROM {$wpdb->terms} AS t 
11   WHERE t.term_id IN (
12    SELECT tt.term_id FROM {$wpdb->term_taxonomy} AS tt
13    WHERE tt.taxonomy = '{$taxonomy_name}'
14   )"
15);
16
17# Deletes the information used by WordPress 
18# to link each post to their terms.
19#
20# Here too, a nested query is needed, 
21# and the 'term_taxonomy' table is yet again 
22# the source of the data passed to the outer query.
23# But in this case to be collected by 
24# the inner query are the ids of 
25# the Relationships to delete.
26$wpdb->query( 
27  "DELETE FROM {$wpdb->term_relationships} AS tr 
28   WHERE tr.term_taxonomy_id IN (
29    SELECT tt.term_taxonomy_id FROM {$wpdb->term_taxonomy} AS tt
30    WHERE tt.taxonomy = '{$taxonomy_name}'
31   )"
32);
33
34# Deletes the base information about the taxonomy. 
35$wpdb->query( 
36  "DELETE FROM {$wpdb->term_taxonomy} 
37   WHERE taxonomy = '{$taxonomy_name}'"
38);

The database must be queried in three distinct times not only because three are the tables from where to delete data, but mainly because two of them, terms and term_relationships, depend on the third, term_taxonomy.