Count(*) in a Subquery with multiple tables: How does SQL determine which table to generate the Coun

I am doing an exercise on subqueries from the Microsoft course # 2071.
There are 3 tables that I need to query in order to return 4 columns:
title_no, title, isbn and Total Reserved. The Total Reserved column is the
per-isbn(book) count of books on reserve with more than 50 reservations and
less than 5 copies of the book.

The tables are as follows
Reservation Title Loan
--------------- ------ --------
isbn title_no isbn
member_no title copy_no
log_date author title_no
log_date synopsis member_no
out_date
due_date

Here's the solution. My question is: How does SQL determing which table's
rows to count?

Select t.title_no, title, l.isbn, count(*) as 'Total Reserved'--how does sql
know which table to count from?
from title AS t
inner join loan AS l
ON t.title_no=l.title_no
inner join reservation r
ON l.isbn=r.isbn
WHERE r.isbn IN
(select isbn
from reservation
group by isbn
having Count(*)>50)
And l.copy_no<5
Group By t.title_no, title, l.isbn

 

Hi Kaimuri,

Imagine that when SQL joins tables it makes one long row out of the matching
rows from the two (or more) tables. Then for "Count(*)" it just counts the
rows. However, if you specify Count(Distinct Tablex.Isbn) then it counts
something more specific.

 

Imagine that when SQL joins tables it makes one long row out of the matching

.... To clarify, where 'tables' mean the resulting tuples from one or
more query parts in the statement.

Too often the term 'table' is used to describe a relation, which is
incorrect.

.... and in the process, excludes non null values from the result.

Kline Sphere (Chalk) MCNGP #3

 

Thanks a bunch..Cindy and Kline!