SQL cross searching two tables

I have the following 2 tables..

mysql> describe catalog_category_reference;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment | 
| name     | text    | NO   |     | NULL    |                | 
| class_id | text    | NO   |     | NULL    |                | 
+----------+---------+------+-----+---------+----------------+

and

mysql> describe product_import_queue;
+-----------------------+---------+------+-----+---------+----------------+
| Field                 | Type    | Null | Key | Default | Extra          |
+-----------------------+---------+------+-----+---------+----------------+
| id                    | int(11) | NO   | PRI | NULL    | auto_increment | 
| unique_id             | text    | NO   |     | NULL    |                | 
| category_code         | text    | NO   |     | NULL    |                | 
| item_code             | text    | NO   |     | NULL    |                | 
| ffl_flag              | int(11) | NO   |     | NULL    |                | 
| name                  | text    | NO   |     | NULL    |                | 
| price                 | text    | NO   |     | NULL    |                | 
| image                 | text    | NO   |     | NULL    |                | 
| custom_options_flag   | int(11) | NO   |     | NULL    |                | 
| custom_options_string | text    | NO   |     | NULL    | 开发者_JAVA技巧               | 
| short_desc            | text    | NO   |     | NULL    |                | 
| long_desc             | text    | NO   |     | NULL    |                | 
| process_status        | int(11) | NO   |     | 0       |                | 
+-----------------------+---------+------+-----+---------+----------------+

I want to search through product_import_queue and find the "category_code"'s that do not exist in the catalog_Category_reference. Please note that, the category_code is stored in the catalog_reference table under class_id. Can I do this in one query? I've attempted something like...

SELECT category_code FROM product_import_queue
LEFT JOIN catalog_category_reference ON product_import_queue.category_code = catalog_category_reference.class_id;

But thats not what I'm looking for and I don't fully understand JOIN's yet.

You were almost there...just need to add a where clause...

SELECT category_code 
FROM   product_import_queue 
       LEFT JOIN catalog_category_reference 
         ON product_import_queue.category_code = 
            catalog_category_reference.class_id 
WHERE  catalog_category_reference.class_id IS NULL  

SELECT category_code FROM product_import_queue
LEFT JOIN catalog_category_reference ON product_import_queue.category_code = catalog_category_reference.class_id
WHERE catalog_category_reference.class_id is null

 SELECT category_code FROM product_import_queue LEFT JOIN catalog_category_reference ON product_import_queue.category_code = catalog_category_reference.class_id WHERE catalog_category_reference.class_id IS NULL

This query will find only products in queque which category_code isn't in category reference