great article Thanks

SELECT books.bk_no

FROM books

INNER JOIN ordered_items

ON books.bk_no = ordered_items.bk_no

GROUP BY books.bk_no

HAVING COUNT(ordered_items.bk_no)>2

This is beause you haven't included the'books.title' in your group by

What you can do is

select * from books

where...

The relationship between those two tables are many-to-many.

You can use select * from contact join (select max(owner_id) from transactions group by owner_id) b

on contact.id = b.owner_id

You can also check...

I agree.

Null is an very interesting data type.

a=null

b=null

if a=b then 'Yes'

Else

'No'

You will expect to get No since you cannot compare two null values.