MySQL CONCAT values on SELF JOIN - mysql

Question

l'm working on a query for an ajax auto suggestion feature on my website and need a little help.
I'm using the following query:
SELECT
CONCAT(b.cat_name, ' > ', a.cat_name) as value,
a.cat_id as data
FROM categories a
LEFT JOIN categories b
ON b.cat_id = a.cat_parent_id
WHERE a.cat_path LIKE '%health%' OR a.cat_slug LIKE '%health%' OR a.cat_name LIKE '%health%' OR a.cat_legacy_path LIKE '%health%'
ORDER BY a.cat_path ASC LIMIT 500
The problem is coming up with root categories where parent_cat_id IS NULL
I'd still like them to appear in my results however as it stands now, the results are coming back empty for the root categories.
Can someone help me tweak this query so that l can accomplish what l'm looking to do?
If it makes any difference l'm doing this with CodeIgniter 3, so here's the actual active record code l'm using on the query:
$search_term = strtolower($search_term);
$this->db->select("CONCAT(b.cat_name, ' > ', a.cat_name) as value, a.cat_id as data");
$this->db->from('categories a');
$this->db->like('a.cat_path', $search_term, 'both', false);
$this->db->or_like('a.cat_slug', $search_term, 'both', false);
$this->db->or_like('a.cat_name', $search_term, 'both', false);
$this->db->or_like('a.cat_legacy_path', $search_term, 'both', false);
$this->db->join('categories b', 'b.cat_id = a.cat_parent_id', 'left');
$this->db->order_by("a.cat_path", 'ASC');
$this->db->limit('500');
$query = $this->db->get();
Ideally l'd like the root categories (where cat_parent_id IS NULL) to just be selected as a.cat_name as value as apposed to CONCAT(b.cat_name, ' > ', a.cat_name) as value.
Any help would be greatly appreciated.

Answer 1

You just need to make it conditional:
SELECT
COALECSE(CONCAT(b.cat_name, ' > ', a.cat_name), a.cat_name) as value,
or, checking more precisely for this exact cause of NULL,
SELECT
IF(a.cat_parent_id IS NULL, a.cat_name, CONCAT(b.cat_name, ' > ', a.cat_name)) as value,
So in your case in codeigniter that would translate to something like:
$this->db->select("COALECSE(CONCAT(b.cat_name, ' > ', a.cat_name), a.cat_name) as value, a.cat_id as data");