Question

This is such a simple problem but for some reason I cannot get my head round it today.

I have two entities:- title and product each respectively named tbl_title and tbl_product. Each title can have many products.

The product table has a field called unwanted which can be either null, 0 or 1.

I wish to select all titles based on where all products (ALL) have unwanted set to 1. So in other words I wish to select the parent based upon all children filling a certain condition. So if a title has one product that is unwanted but another that is not I do not wish for this title to enter the result set.

When I try this the most I get out of my head is:

SELECT * FROM `tbl_title` 
    left join tbl_product on tbl_product.title_id = tbl_title.id 
    where tbl_product.unwanted = 1 
    group by tbl_title.id

Which obviously does not work.

So how do I code such a query?

Answer 1

select * from tbl_title
where id not in (select title_id from tbl_product where unwanted = 0)

In English, this query eliminates all titles that have a wanted product.

From a style point of view, it would be better to call your column wanted, because unwanted = 0 is a double-negative of wanted = 1. It’s always easier to get your head around positives.