Question

I want to select everything from the product table where the name is not equal to a name existing in ordered_products. I tried this but it didnt work:

SELECT * FROM product WHERE naam IS NOT (SELECT naam FROM ordered_products)

Answer 1

You can use the NOT IN predicate like so:

SELECT * 
FROM product 
WHERE naam NOT IN (SELECT naam 
                   FROM ordered_products
                   WHERE naam IS NOT NULL);

But this is not safe, in case there is any NULL values of the naam coming from the other table, thats why I added WHERE naam IS NOT NULL to the subquery. Better off, use JOIN like so:

SELECT p.* 
FROM product
LEFT JOIN ordered_products o ON p.naam = o.naam
WHERE p.naam IS NULL;