How would you explain this query in layman terms? - sql

Question

Here is the database I'm using: https://drive.google.com/file/d/1ArJekOQpal0JFIr1h3NXYcFVngnCNUxg/view?usp=sharing
select distinct
AC1.givename, AC1.famname, AC2.givename, AC2.famname
from
academic AC1, author AU1, academic AC2, author AU2
where
AC1.acnum = AU1.acnum
and AC2.acnum = AU2.acnum
and AU1.panum = AU2.panum
and AU2.acnum > AU1.acnum
and not exists (select *
from Interest I1, Interest I2
where I1.acnum = AC1.acnum
and I2.acnum = AC2.acnum);
Output:
I'm having trouble explaining this output of the subquery and query in layman terms(Normal english).
Not sure if my explanation is right:
"The subquery finds the interested fields where two authors have no common field of interest.
The whole query finds the first and last names of the authors of papers which have at least two authors, and have no common field of interest."

Answer 1

As it currently stands, the subquery will produce rows if each academic has at least one interest.
So overall, the query is "produce pairs of academics who co-authored at least one paper and where at least one of them has no interests whatsoever". It's difficult to believe that that was the intent, and if it was, there are clearer ways of writing it that make it more clear that that is what we're looking for.
If that's the query we want, though, I'd write it as:
SELECT
AC1.givename, AC1.famname, AC2.givename, AC2.famname
FROM
academic AC1
inner join
academic AC2
on
AC1.acnum < AC2.acnum
WHERE EXISTS
(select * from author au1 inner join author au2 on au1.panum = au2.panum
where au1.acnum = ac1.acnum and au2.acnum = ac2.acnum)
AND
(
NOT EXISTS (select * from interest i where i.acnum = ac1.acnum)
OR
NOT EXISTS (select * from interest i where i.acnum = ac2.acnum)
)
If, as is more likely, we wanted pairs of co-authors who have no interests in common, we would write something like:
SELECT
AC1.givename, AC1.famname, AC2.givename, AC2.famname
FROM
academic AC1
inner join
academic AC2
on
AC1.acnum < AC2.acnum
WHERE EXISTS
(select * from author au1 inner join author au2 on au1.panum = au2.panum
where au1.acnum = ac1.acnum and au2.acnum = ac2.acnum)
AND NOT EXISTS
(select * from interest i1 inner join interest i2 on i1.field = i2.field
where i1.acnum = ac1.acnum and i2.acnum = ac2.acnum)
Notice how neither of my queries uses distinct, because we've made sure that the outer query isn't joining additional rows where we only care about the existence or absence of those rows - we've moved all such checks into EXISTS subqueries.
I generally see distinct used far too often when the author is getting multiple results when they only want a single result and they're unwilling to expend the effort to discover why they're getting multiple results. In this case, it would be situations where the same pairs of academics have co-authored more than one paper.