Beginner SQL trouble with simple Join/Subquery - sql

Question

I know this is a quite a basic query, but I can't find exactly the solution for my needs, so here goes...
I have a table of customers INT_AUX_LISTING, a table of folders in which they're contained INT_AUX_DIRECTORY and a joining table INT_AUX_DIR_LIST.
I need to return a list of all customers who are in folder 40017, and also not in folder 2. Any other folder in which they're contained is irrelevant.
So far I've come up with
SELECT *
FROM INT_AUX_LISTING l
LEFT JOIN INT_AUX_DIR_LIST dl
ON l.LISTING_ID=dl.LISTING_ID
WHERE dl.CONTAIN_DIR_ID=40017
AND dl.CONTAIN_DIR_ID <> 2
However this (obviously) isn't correct, and is returning far too many values. I think I need to introduce a subquery but things go awry when I try.
Apologies for the entry level nature of the question, but it's only my 3rd day working with SQL!

Answer 1

First, clarification on left-join and where. If you have a query that has a left-join to a table (per your example where "dl" was the right-side of the join), and then add the "dl" alias to a where clause, that in essence converts it to an INNER JOIN (unless you explicitly tested with an OR dl.listing_ID IS NULL).
Now, back to your primary objective. Without the table structures, and not exactly clear on your column intent, it APPEARS that the "listing_id" column is your client as found in the int_aux_listing table. the "contain_dir_id" is the directory (one of many that may be possible) as found in the int_aux_dir_list.
That said, you want a list of all people who are in 40017 AND NOT in 2. For this, you can do a left-join but using the int_aux_listing table twice but with different aliases. Once you get the list of clients, you can then join that to your client table. Since you are new, take the sample one step at a time.
Just get clients within 40017
select
dl.listing_id
from
int_aux_dir_list dl
where
dl.contain_dir_id = 40017
Now, exclude those if they are found in directory 2
select
dl.listing_id
from
int_aux_dir_list dl
left join int_aux_dir_list dl2
on dl.listing_id = dl2.listing_id
AND dl2.contain_dir_id = 2
where
dl.contain_dir_id = 40017
AND dl2.listing_id IS NULL
By doing a left-join to the directory listing table a second time (via alias "dl2"), but specifically for its directory id = 2 (via AND dl2.contain_dir_id = 2), you are getting all clients from dl that are within directory 40017 and looking for that same client ID in the second instance ("dl2").
Here's the kicker. Notice the where clause is stating "AND dl2.listing_id IS NULL".
This is basically saying WHEN Looking at the DL2 instance, I only want records that DO NOT have a corresponding listing ID matched.
Now, wrap this up to get the client information you need. In this case, getting all columns from the directory listing (in case other columns of importance), AND getting all columns from the "l"isting table.
select
dl.*,
l.*
from
int_aux_dir_list dl
left join int_aux_dir_list dl2
on dl.listing_id = dl2.listing_id
AND dl2.contain_dir_id = 2
join INT_AUX_LISTING l
on dl.listing_id = l.listing_id
where
dl.contain_dir_id = 40017
AND dl2.listing_id IS NULL
Hopefully this clears things up for you some.

Answer 2

Your WHERE clause is the problem; it brings back ALL rows except the one where that id is not 2, including the one where id is 40017 (since it's not equal to 2). I think you should revisit it.

Answer 3

If you use a LEFT JOIN[1] on INT_AUX_LISTING, you will get all customers even if they don't have a corresponding folder. In your case it's best to use INNER JOIN[2] instead.
In the where clause, you're first statement selects customers who are in folder 40017 (dl.CONTAIN_DIR_ID=40017), because of this your second statement (dl.CONTAIN_DIR_ID <> 2) will always be true (40017 will never be equal to 2). So basically your selecting all customers in folder 40017.
Your notion of a subquery was correct. This is one way of getting the desired result:
SELECT *
FROM INT_AUX_LISTING l
INNER JOIN INT_AUX_DIR_LIST dl1
ON l.LISTING_ID = dl1.LISTING_ID
WHERE dl1.CONTAIN_DIR_ID = 40017
AND l.LISTING_ID NOT IN (
SELECT dl2.LISTING_ID FROM INT_AUX_DIR_LIST dl2 WHERE dl2.CONTAIN_DIR_ID = 2
)