SQL query to find the faculty which have taught every subject

Question

I need to write a SQL query to find the faculty which has taught every subject (ie Sam)

1. With nested queries
2. Without using aggregate functions (no count, avg, min etc).

I can't seem to figure this out, would really appreciate some help =)

Faculty

fid	fname	fqualifications	fexperience	salary	deptname
100	Sam	ME CS	10	100000	IT
101	John	ME IT	8	80000	IT
102	Max	ME CS	9	90000	CS
103	Jenny	ME CS	5	50000	CS

Course

cid	cname	semester
1	SE	4
2	WT	4
3	CG	5
4	DBMS	5

Teaches

fid	cid	year
100	1	2019
100	2	2018
100	3	2020
100	4	2021
101	1	2017
101	2	2018
102	2	2018
102	3	2019
103	3	2020
103	4	2021

I used this query to find the output but according to the question I can't.

select * from faculty f
    -> inner join teaches t
    -> on f.fid=t.fid
    -> inner join course c
    -> on t.cid=c.cid
    -> group by f.fid,f.fname
    -> having count(*)=4;

OUTPUT:

fid	fname	fqualifications	fexperience	salary	deptname	fid	cid	year	cid	cname	semester
100	Sam	ME CS	10	100000	IT	100	1	2019	1	SE	4

Answer 1

Not the most efficient way to proceed, but with the requirements given to you, I would try and rephrase the query like this:

"A faculty that has taught every subject is a faculty that has not skipped even one subject".

Now, faculties that have skipped a subject will have a NULL when LEFT JOINed with the syllabus and all the subjects. Pseudo-SQL:

SELECT DISTINCT faculty.id FROM faculties 
    LEFT JOIN has_taught ON (has_taught.faculty_id = faculty.id)
    LEFT JOIN subjects ON (has_taught.subject_id = subjects.id)
    WHERE has_taught.faculty_id IS NULL;

or in some databases you maybe need

SELECT DISTINCT faculty.id FROM faculties 
    CROSS JOIN subjects
    LEFT JOIN has_taught ON 
      (has_taught.faculty_id = faculty.id AND has_taught.subject_id = subjects.id)
    WHERE has_taught.faculty_id IS NULL;

So, faculties that are NOT IN this list would naturally be

SELECT * FROM faculties
    WHERE faculty.id NOT IN (
        SELECT DISTINCT faculty.id ...
    );

and this should only use nested queries, as requested.

Or with a further join

SELECT faculties.* FROM faculties
    LEFT JOIN (
        SELECT DISTINCT faculty.id ...
    ) AS they_skipped_some
    ON (they_skipped_some.id = faculties.id)
    WHERE they_skipped_some.id IS NULL