Filtering sqlite query - sql

Question

Having trouble with filtering in my sqlite3 query. I am working with three tables below.
Table: models
id|data
1|car
2|truck
Table: descriptions
id|model_id|colour|make
1|1|blue|accord
2|1|green|prius
3|1|red|fusion
4|1|black|civic
5|1|white|jeep
6|1|purple|jeep
7|1|brown|jeep
8|1|brown|civic
Table: banned
model_id|colour_id|colour
1|3|black|
1|15|brown|
The below statement is counting how many of what model (cars or trucks) are what make.
SELECT models.id, make, count(make)
FROM description
JOIN models ON models.id = descriptions.model_id
GROUP BY models.id, descriptions.make;
The output would below
1|accord|1
1|prius|1
1|fusion|1
1|civic|2
1|jeep|3
However, I want to put in a qualifier that voids anything containing a banned colour/model combo, by using banned.colour.
I tried joining the table and filtering out like below, but it seems to double the count.
SELECT models.id, make, count(make)
FROM description
JOIN models ON models.id = descriptions.model_id
JOIN banned ON banned.model_id = models.id
WHERE NOT ( banned.colour = descriptions.colour)
GROUP BY models.id, descriptions.make;
My desired output is to void the two cars that fit this qualifier from the count. The final result should below
1|accord|1
1|prius|1
1|fusion|1
1|jeep|2
How can i achieve this?

Answer 1

You can use your approach . . . with a left join and where check:
SELECT m.id, d.make, count(*)
FROM description d JOIN
models m
ON m.id = d.model_id LEFT JOIN
banned b
ON b.model_id = m.id AND b.colour = d.colour
WHERE b.model_id IS NULL -- no match
GROUP BY m.id, d.make;
A common way to write the query would also use NOT EXISTS:
SELECT m.id, d.make, count(*)
FROM description d JOIN
models m
ON m.id = d.model_id
WHERE NOT EXISTS (SELECT 1
FROM banned b
WHERE b.model_id = m.id AND b.colour = d.colour
)
GROUP BY m.id, d.make;
Although you can also use NOT IN, I highly discourage using it with a subquery. It will not do what you want if any of the values returned by the subquery are NULL.

Answer 2

You may use NOT IN with a dependent subquery to avoid banned combinations.
SELECT models.id, make, count(make)
FROM description
JOIN models ON models.id = descriptions.model_id
WHERE colour NOT IN (
SELECT colour
FROM banned
WHERE banned.model_id = models.id
)
GROUP BY models.id, descriptions.make;
The only pitfall of NOT IN is a possibility to have a NULL in the subquery result. However, I believe that it is safe if the colour attribute is defined as NOT NULL (which make sense in this case).