Need SQL help: Selecting a list of categories that don't have subcategories underneath them

I have 2 tables in the database - categories and subcategories

In the categories table, I have an ID column (primary key) and another column for the category name.

In the subcategories table, I have an ID column (primary key), .. a column for the subcategory name, .. and another column that stores the name of the parent category that the subcategory has been assigned to.  

categories = ID, category
subcategories = ID, subcategory, parent_category

I am now trying to form a query that selects the names of all parent categories (from the categories table) which do NOT have subcategories assigned to them, .. by somehow comparing it against the subcategories table.    

The following query does exactly the OPPOSITE of what I want it to do (it returns all of the parent categories from the categories table that DO have subcategories assigned to them) :

SELECT DISTINCT categories.category, categories.ID AS catid
FROM categories,subcategories
WHERE categories.category = subcategories.parent_category
ORDER BY categories.category ASC

I tried using the "=" operator as one of my troubleshooting steps, .. since using "<>" and "!=" didn't work for me (using either of those operators returns all categories from the categories table, regardless as to whethe or not there are subcategories assigned).  

Could someone here please help me write a query that selects the names of all parent categories (from the categories table) which do NOT have subcategories assigned to them?

Thanks in advance,
- Yvan