SQL query for wp_links in multiple categories - php

Question

I can get links from the WordPress database that are in a specific category like this (I have added line breaks to the SQL part for clarity):
$category1 = 'stuff';
$category2 = 'other_stuff';
$q = 'select * from wp_links l
inner join wp_term_relationships r on l.link_id = r.object_id
inner join wp_term_taxonomy using (term_taxonomy_id)
inner join wp_terms using (term_id)
where taxonomy = "link_category"
and name = '.$category1;
$results = $wpdb->get_results($q);
How would I retrieve links that are in both $category1 and $category2 (I do mean both categories, not either category)?

Answer 1

On the assumption that the name field you are searching on is in the wp_terms table, there are two principle options.
The first just looks for entities with the first category, then uses another join the find those that also have the second category. This involes using the same table in two different joins, and so uses aliases and avoids the using keyword.
select * from wp_links l
inner join wp_term_relationships r on l.link_id = r.object_id
inner join wp_term_taxonomy t on r.term_taxonomy_id = t.term_taxonomy_id
inner join wp_terms c1 on t.term_id = c1.term_id
inner join wp_terms c2 on t.term_id = c2.term_id
where taxonomy = "link_category"
and c1.name = 'stuff'
and c2.name = 'other_stuff'
The alternative is much more scalable to more than just two catagories, but involves a sub-query...
select
l.*
from
(
select l.id from wp_links l
inner join wp_term_relationships r on l.link_id = r.object_id
inner join wp_term_taxonomy t on r.term_taxonomy_id = t.term_taxonomy_id
inner join wp_terms c on t.term_id = c.term_id
where taxonomy = "link_category"
and c.name IN ('stuff', 'other_stuff')
group by l.id
having count(distinct c.name) = 2
)
subquery
inner join wp_links l ON subquery.id = l.id
The inner query finds all that have one category or the other, but then the having clause only lets through those that have two categories in our list. (In other words, both of them.) [It also assumes that the wp_links table as an id column to use as a unique identifier.]

Answer 2

did you try with the OR?
$q = 'select * from (select * from wp_links l
inner join wp_term_relationships r on l.link_id = r.object_id
inner join wp_term_taxonomy using (term_taxonomy_id)
inner join wp_terms using (term_id)
where taxonomy = "link_category"
and name = '.$category1.') as t1 join (select * from wp_links l
inner join wp_term_relationships r on l.link_id = r.object_id
inner join wp_term_taxonomy using (term_taxonomy_id)
inner join wp_terms using (term_id)
where taxonomy = "link_category"
and name = '.$category2 .') as t2
on t1.term_id=t2.term_id;
I don't think this is the best solution, but you don't give me more details about your structure, tables what you have