MySQL lookup joining 3 tables

MySQL lookup joining 3 tables

I have 3 tables, location, lookup and exclude with the following info...
location table
+----+--------------+
| id | location |
+----+--------------+
| 1 | Location a |
| 2 | Location b |
| 3 | Location c |
| 5 | Location d |
| 6 | Location e |
| 7 | Location f |
| 8 | Location g |
| 9 | Location h |
+----+--------------+
lookup table
+----+-------------+------+
| id | location_id | code |
+----+-------------+------+
| 1 | 2 | PR6 |
| 2 | 2 | PR7 |
| 3 | 2 | PR9 |
| 4 | 5 | WA2 |
| 6 | 8 | WA3 |
+----+-------------+------+
exclude table
+----+-------------+------+
| id | location_id | code |
+----+-------------+------+
| 1 | 2 | PR5 |
| 2 | 2 | PR8 |
+----+-------------+------+
These tables exist for a basic postcode lookup. I need to create some SQL
so a search can be done using the postcode but I need to take into account
any excluded postcodes so for example... If I was to search for "PR7" I
would get "Location B" as the result but if I was to search for PR5 or
PR8, I would not get "Location B" as these are in the exclude table. Here
is what I have so far....
SELECT
lookup.*, exclude.`code` as exclude, location.location
FROM lookup
LEFT JOIN
exclude
ON lookup.location_id = exclude.location_id
LEFT JOIN
location
ON location.location = lookup.location_id
WHERE lookup.`code` LIKE 'PR%' AND (exclude.`code` NOT LIKE 'PR8%' OR
ISNULL(exclude.`code`))
GROUP BY location.location
ORDER BY location.id
In my SQL above, the user typed "PR8" which is being sent to the where
clause using PHP variables, I look for the 1st 2 letters of the postcode
in the lookup table and then I am also trying to look for what the user
entered in the exclude table to omit this from the results. Unfortunately
I cant tell where I am going wrong, I am getting the following results
where as based on the above tables I should be getting no results due to
the exclusion...
+----+-------------+------+---------+
| id | location | code | exclude |
+----+-------------+------+---------+
| 1 | Location b | PR8 | PR5 |
+----+-------------+------+---------+
I hope I have explained this well enough but please tell me if I need to
add more detail.
I would really appreciate some guidance on this, thanks so much in advance :)