Sqlite SELECT with multiple conditions - sqlite

Question

I have to create a database with a PRODUCTS table and a CATEGORIES table.
Each product has a name, a price, a creation date and may belong to several categories.
Categories have a name and a flag to indicate whether the category is private or public.
Then I have to select all records that belongs to more than 5 public categories.
I've created the tables like this:
CREATE TABLE PRODUCTS (
ID_PROD int NOT NULL PRIMARY KEY,
NAME TEXT(255),
PRICE INTEGER,
CREATION_DATE DATE
);
CREATE TABLE CATEGORIES (
ID_CAT INTEGER NOT NULL PRIMARY KEY,
NAME TEXT(255),
PRIVATE INTEGER
);
CREATE TABLE PROD_CAT (
ID INTEGER NOT NULL PRIMARY KEY,
ID_PROD INTEGER,
ID_CAT INTEGER,
FOREIGN KEY (ID_PROD) REFERENCES PRODUCTS(ID_PROD),
FOREIGN KEY (ID_CAT) REFERENCES CATEGORIES(ID_CAT)
)
I've managed to select all the records that belongs to more than 5 categories but I can't find out how to add the public category condition...
Here's what I've tried:
This works:
SELECT NAME
FROM PRODUCTS
WHERE ID_PROD IN (SELECT ID_PROD FROM PROD_CAT GROUP BY ID_PROD HAVING COUNT(*)>5)
But not this:
SELECT PRODUCTS.NAME
FROM PRODUCTS, CATEGORIES
WHERE ID_PROD IN (SELECT ID_PROD FROM PROD_CAT GROUP BY ID_PROD HAVING COUNT(*)>5)
AND CATEGORIES.PRIVATE = 1
Any help would be appreciated :)

Answer 1

You need a join of PROD_CAT to CATEGORIES:
SELECT NAME
FROM PRODUCTS
WHERE ID_PROD IN (
SELECT pc.ID_PROD
FROM PROD_CAT pc INNER JOIN CATEGORIES c
ON c.ID_CAT = pc.ID_CAT
WHERE c.PRIVATE -- or WHERE NOT c.PRIVATE for public categories
GROUP BY pc.ID_PROD
HAVING COUNT(*) > 5
)
Or, without the operator IN, with joins of all 3 tables:
SELECT p.ID_PROD, p.NAME
FROM PRODUCTS p
INNER JOIN PROD_CAT pc ON pc.ID_PROD = p.ID_PROD
INNER JOIN CATEGORIES c ON c.ID_CAT = pc.ID_CAT
WHERE c.PRIVATE -- or WHERE NOT c.PRIVATE for public categories
GROUP BY p.ID_PROD, p.NAME
HAVING COUNT(*) > 5

Answer 2

Since this looks like homework, I'll give a good hint.
Your first query returns products belonging to more than 5 categories, using a sub-query for the COUNT. The restriction you added in the second query was added to the top-level WHERE-clause, not the sub-query. The sub-query still works on PROD_CAT and still returns the same results, which may include public categories.