MySQL LEFT JOIN where joined table is null - php

Question

My current SQL is:
SELECT * FROM `node`
LEFT JOIN `field_data_field_area_of_study_category`
ON `field_data_field_area_of_study_category`.`entity_id` = `node`.`nid`
WHERE `node`.`type` = 'area_of_study'
GROUP BY `node`.`nid`
For now, to make it work I run a foreach loop to is_null() on the column.
I'm trying to emulate the is_null() check within the SQL query.
Thanks for reading.
EDIT
This is the foreach loop that runs the is_null(). I think a better way to ask my question would be: how do make the SQL return only rows from the node table where there is no matching field_data_field_area_of_study_category table?
foreach($aos_nodes as $aos_node):
if (is_null($aos_node->field_area_of_study_category_tid)):
$menu_item_display[$aos_node->title] = array(
'id' => $aos_node->nid,
'type' => 'node',
'children_markup' => '',
'icon' => '',
'name' => $aos_node->title,
'href' => drupal_get_path_alias('node/'.$aos_node->nid),
);
endif;
endforeach;

Answer 1

To return the rows where there isn't a matching one:
SELECT * FROM `node`
LEFT JOIN `field_data_field_area_of_study_category`
ON `field_data_field_area_of_study_category`.`entity_id` = `node`.`nid`
WHERE `node`.`type` = 'area_of_study'
and `field_data_field_area_of_study_category`.`entity_id` is null
GROUP BY `node`.`nid`
Additionally, you can use not exists:
select
*
from
node n
where
n.type = 'area_of_study'
and not exists (
select
1
from
field_data_field_area_of_study_category f
where
f.entity_id = n.nid
)