Adv DBS - Set Predicates using Count

-- QUERY 1
-- Find sids of who bought no books (various formulations)

-- Using set difference EXCEPT

(select s.sid
 from   student s)
except
(select t.sid
 from   buys t);

-- Using NOT IN predicate

select s.sid
from   student s
where s.sid not in (select t.sid
                     from   buys t);

-- Using <> ALL predicate

select s.sid
from   student s
where  s.sid <> ALL(select t.sid
                    from   buys t);

-- Using NOT EXISTS predicate

select s.sid
from   student s
where  not exists( select t.bookno
                   from   buys t
                   where  t.sid = s.sid);

-- Using the COUNT aggregate function

select s.sid
from   student s
where  (select count(t.bookno)
        from   buys t
        where  t.sid = s.sid) = 0;

-- With a user-defined function

create or replace function size_of_books_bought_by_student(s int) returns bigint AS 
$$
  select count(t.bookno)
  from   buys t
  where  t.sid = s;
$$ language sql;

select s.sid 
from   student S 
where  size_of_books_bought_by_student(s.sid) = 0;

---

-- QUERY 2
-- Find sids of student who bought at least at 4 books

-- Using the counting method

select s.sid
from   student s
where  (select count(t.bookno)
        from   buys t
        where  t.sid = s.sid) >= 4;

-- With user-defined function

select s.sid 
from   student S 
where  size_of_books_bought_by_student(s.sid) >=  4;

---

-- QUERY 3
-- Find the sids of students who buy only books that cost more than $50

-- Strategy:
-- a. Consider the set BooksBoughtBySTudent(sid) = {bookno| buys(sid,bookno}
-- i.e. the set of books bought by student sid

-- This set can be specified with a sub-query

-- select t.bookno
-- from buys t
-- where t.sid = s.sid -- s is a variable ranging over the student table

-- b. Then consider the set of books that cost more than $50. Call this set Book_less_than_50
-- This set can be specified using a query

-- select b.bookno
-- from book b
-- where b.price > 50

-- The query can now be formulated as follows
-- {sid | BooksBoughtBySTudent(sid) is a subset of Book_more_than_50}

-- or equivalently with counting,
-- {sid | count(BoughtBoughtbyStudent(sid) - Book_more_than_50) = 0}

-- We can put all of this together and write the query

select s.sid
from   student s
where  (select count(1) from  (select t.bookno
                               from   buys t
                               where  t.sid = s.sid and
                                     (select count(1)
                                      from   book b
                                      where  b.price > 50 and t.bookno = b.bookno) >= 1) q) = 0;

-- Or, equivalently, without using the EXCEPT operator

select s.sid
from   student s
where  (select count(1) from ((select t.bookno from buys t where t.sid = s.sid)
                                except 
                              (select b.bookno from book b where b.price > 50)) q) = 0;

---

-- QUERY 4
-- FInd the sids of students that buy all books that cost more that $50
-- Notice the similarity with QUERY 3: we just had to interchange the inner
-- two sets around the EXCEPT operator.

select s.sid
from   student s
where   (select count(1) from ((select b.bookno from book b where b.price > 50)
                               except
                               (select t.bookno from buys t where t.sid = s.sid)) q) = 0;

---

-- QUERY 5
-- FInd the sids of students that buy at least 3 books that cost more that $20

select s.sid
from   student s
where   (select count(1) from ((select t.bookno from buys t where t.sid = s.sid)
                               intersect
                               (select b.bookno from book b where b.price > 20)) q) >= 3;

---

-- QUERY 6 ***
-- Find the pairs of student sid (s1,s2) such that student s1 buys all but 3 books that
-- are bought by student s2.
-- So know we need to consider the sets
-- BooksBoughtByStudent(s1) and BooksBoughtByStudent(s2)
--
-- We then need to say that student s2 buys exactly 3 books that are not bought by
-- student s1.
-- In other words count(BooksBoughtByStudent(s2) - BooksBoughtByStudent(s1)) = 3.
-- So we formulate this query as follows:

select s1.sid, s2.sid
from   student s1, student s2
where  (select count(1) from ((select t2.bookno from buys t2 where t2.sid = s2.sid)
                               except
                              (select t1.bookno from buys t1 where t1.sid = s1.sid)) q) = 3;