Question

I have several tables, books, bookcategories, categories

bookcategories is a join table to allow the many to many relationship between books and categories.

I want to be able to run a category search on books such that the search returns one row per book even when the book has many categories.

Books

ID   | Title
1    | Once upon...
2    | How many...
3    | How much...

Categories

ID   | Category
1    | x
2    | y
3    | z

BookCategories

   BookId | CategoryId
     1    |     1
     1    |     2
     2    |     2
     2    |     3
     2    |     1
     3    |     1

I thought I could get away with this:

SELECT  b.Id,
    b.Title,
FROM    ( books b
    INNER JOIN bookcategories bc ON b.ID= bc.BookId
    )
WHERE (bc.categoryId =1 AND bc.categoryId=2)
GROUP BY b.Id, b.Title

but as soon as I add the AND, the query returns no rows. But that is the criteria I need to apply – I only want to return book rows where the book has both category 1 and 2 as above (i.e. not category 1 OR category 2)

Can’t help thinking that I am missing something quite basic. Can anyone help?

Do I need to change the structure of the tables or is there a way of achieving what I need.

Wing

Answer 1

This should return books that have both category one and two:

select  b.Id
,       b.Title
from    books b
join    bookcategories bc
on      bc.BookId = b.Id
group by
         b.Id
,        b.Title
having   sum(case when bc.categoryId = 1 then 1 end) > 0
         and sum(case when bc.categoryId = 2 then 1 end) > 0

Alternatively, you can use a double exists clause:

select  b.Id
,       b.Title
from    books b
where   exists
        (
        select  *
        from    bookcategories bc
        where   bc.BookId = b.Id
                and bc.CategoryId = 1
        )
        and exists
        (
        select  *
        from    bookcategories bc
        where   bc.BookId = b.Id
                and bc.CategoryId = 2
        )