MySQL excluding results on a JOIN - mysql

Question

I'm working on a restaurant CMS app. I have a many-to-many relationship between 2 tables, menu_sections and menu_items. The relationship is maintained with a table in between called menu_relationships.
As an example let's say the menu section called Snacks (menu_section_id = 1) contains a menu item called Pretzels (menu_item_id = 1) and the menu section called Desserts (menu_section_id = 2) contains a menu item called Ice Cream (menu_item_id = 2), but Ice Cream is also contained within another menu section called Kids Food (menu_section_id = 3). So there would be 3 rows in the menu_relationships table to map out these 3 relationships. The relationship table would look like this:
---------------------------------------
| menu_section_id | menu_item_id |
|=====================================|
| 1 | 1 |
|-------------------------------------|
| 2 | 2 |
|-------------------------------------|
| 3 | 2 |
---------------------------------------
So far so good.
I want to generate a result set that will return the names of all menu items except for menu items with a given menu_section_id. So to return the menu item names, I have a join on the menu_items table. Here's the SQL:
SELECT menu_section_id, menu_items.menu_item_id, menu_item_name
FROM menu_relationships
JOIN menu_items
ON menu_items.menu_item_id = menu_relationships.menu_item_id
WHERE menu_section_id != 2
The result set which will give me a row for each relationship that doesn't contain a given menu_section_id. With the example data I would be getting 2 rows back from the relationship table:
-----------------------------------------------------------
| menu_section_id | menu_item_id | menu_item_name |
|======================================|==================|
| 1 | 1 | Pretzels |
|--------------------------------------|------------------|
| 3 | 2 | Ice Cream |
-----------------------------------------------------------
But what I want is to exclude the menu item altogether from the result set, if it has ANY relationship to the specified menu_section_id. In other words, in the case of this example , I only want to return rows for menu items that have no relationship mappings at all to a menu_section_id of 2, I only want to return the Pretzels row.
I've tried various things with GROUP BY and HAVING using the bit_xor() aggregate function, but so far no luck at all in getting what I want.
I probably could have taken less time to explain that but I wanted it to be a clear as I can make it. I hope it is. Can anyone help?

Answer 1

This is a wonderful case for the use of LEFT OUTER JOIN because it includes all rows from your left-hand table and matches where it can, returning NULL for any non-match.
Building on Mark Breyer's sample query from above, see this example:
SELECT R.menu_section_id, I.menu_item_id, I.menu_item_name
FROM menu_items AS I
LEFT OUTER JOIN menu_relationships R on (R.menu_item_id=I.menu_item_id) AND (R.menu_section_id = 2)
The mysql optimizer may actually rewrite this as a subquery - i'm not an optimization expert by any means - I'd take a look at the way your indexes are built and see if this type of join makes sense for your schema. I'd also test to see if it's actually faster because it's actually less semantic.

Answer 2

There are many ways to do this. Here is one example using WHERE ... NOT IN (...):
SELECT
R.menu_section_id,
I.menu_item_id,
I.menu_item_name
FROM menu_items AS I
JOIN menu_relationships AS R
ON R.menu_item_id = I.menu_item_id
WHERE I.menu_item_id NOT IN
(
SELECT menu_item_id
FROM menu_relationships
WHERE menu_section_id = 2
)

Answer 3

I would use a subquery for this, getting me every menu_item_id which has the menu_section_id 2 and then using NOT IN. Here you go:
SELECT menu_section_id, menu_items.menu_item_id, menu_item_name
FROM menu_relationships
JOIN menu_items
ON menu_items.menu_item_id = menu_relationships.menu_item_id
WHERE menu_relationships.menu_item_id NOT IN (
SELECT menu_item_id
FROM menu_relationships
WHERE menu_section_id = 2
);

Answer 4

I was going to suggest a subquery a well, except that I wanted to mention that subqueries can dramatically affect performance on your site. You may want to consider options for caching to avoid serious load time hangups due to things like this.
In most cases you'll be ok, but if you're only showing us part of the issue and just not mentioning the irrelevant details then you could very well be building a site where you run 100 of these queries on a page, for example, because someone mentioned it here without mentioning the compounded overhead things like this can result in...
Like I said though, you'll probably be fine. Just don't do a subquery within a subquery unless you want to restart your server.