How to find equal subsets?

Question

I have a table with subsets. How to find reader id's with the same subsets as given id? For example:

Input reader = 4 The expected output: reader 1 and 5.

Subsets size is not always = 3 as in the example it can be dynamic. What is correct SQL query?

declare @t table(
   reader int not null,
   book int,
   pages int
)
insert into @t (reader, book, pages)
select 1, 1, 100 union
select 1, 2, 201 union
select 1, 3, 301 union
select 2, 1, 100 union
select 2, 3, 101 union
select 2, 3, 301 union
select 3, 1, 100 union
select 3, 2, 101 union
select 3, 3, 301 union
select 4, 1, 100 union
select 4, 2, 201 union
select 4, 3, 301 union
select 5, 1, 100 union
select 5, 2, 201 union
select 5, 3, 301

select * from @t

Answer 1

This is a bit of a pain, but you can use a self-join:

with t as (
      select t.*, count(*) over (partition by reader) as cnt
      from @t t
     )
select t.reader
from t left join
     t t2
     on t2.book = t.book and
        t2.pages = t.pages and
        t2.cnt = t.cnt and
        t2.reader = 4
group by t.reader, t.cnt
having count(*) = t.cnt and
       count(*) = count(t2.reader);

The left join is needed to avoid a subsetting relationship. That is, having all the books for "4" plus additional books.