SQL query for querying counts from a table - sql

Question

The prompt is to form a SQL query.
That finds the students name and ID who attend all lectures having ects more than 4.
The tables are
CREATE TABLE CLASS (
STUDENT_ID INT NOT NULL,
LECTURE_ID INT NOT NULL
);
CREATE TABLE STUDENT (
STUDENT_ID INT NOT NULL,
STUDENT_NAME VARCHAR(255),
PRIMARY KEY (STUDENT_ID)
)
CREATE TABLE LECTURE (
LECTURE_ID INT NOT NULL,
LECTURE_NAME VARCHAR(255),
ECTS INT,
PRIMARY KEY (LECTURE_ID)
)
I came up with this query but this didn't seem to work on SQLFIDDLE. I'm new to SQL and this query has been a little troublesome for me. How would you query this?
SELECT STUD.STUDENT_NAME FROM STUDENT STUD
INNER JOIN CLASS CLS AND LECTURE LEC ON
CLS.STUDENT_ID = STUD.STUDENT_ID
WHERE LEC.CTS > 4
How do I fix this query?
UPDATE
insert into STUDENT values(1, 'wick', 20);
insert into STUDENT values(2, 'Drake', 25);
insert into STUDENT values(3, 'Bake', 42);
insert into STUDENT values(4, 'Man', 5);
insert into LECTURE values(1, 'Math', 6);
insert into LECTURE values(2, 'Prog', 6);
insert into LECTURE values(3, 'Physics', 1);
insert into LECTURE values(4, '4ects', 4);
insert into LECTURE values(5, 'subj', 4);
insert into SCLASS values(1, 3);
insert into SCLASS values(1, 2);
insert into SCLASS values(2, 3);
insert into SCLASS values(3, 1);
insert into SCLASS values(3, 2);
insert into SCLASS values(3, 3);
insert into SCLASS values(4, 4);
insert into SCLASS values(4, 5);

Answer 1

The following approach might get the job done.
It works by generating two subqueries :
one that counts how many lectures whose ects is greater than 4 were taken by each user
another that just counts the total number of lectures whose ects is greater than 4
Then, the outer query filters in users whose count reaches the total :
SELECT x.student_id, x.student_name
FROM
(
SELECT s.student_id, s.student_name, COUNT(DISTINCT l.lecture_id) cnt
FROM
student s
INNER JOIN class c ON c.student_id = s.student_id
INNER JOIN lecture l ON l.lecture_id = c.lecture_id
WHERE l.ects > 4
GROUP BY s.student_id, s.student_name
) x
CROSS JOIN (SELECT COUNT(*) cnt FROM lecture WHERE ects > 4 ) y
WHERE x.cnt = y.cnt ;

Answer 2

As GMB already said in their answer: count required lections and compare with those taken per student. Here is another way to write such query. We outer join classes to all lectures with ECTS > 4. Analytic window functions allow us to aggregate by two different groups at the same time (here: all rows and student's rows).
select *
from student
where (student_id, 0) in -- 0 means no gap between required and taken lectures
(
select
student_id,
count(distinct lecture_id) over () -
count(distinct lecture_id) over (partition by c.student_id) as gap
from lecture l
left join class c using (lecture_id)
where l.ects > 4
);
Demo: https://dbfiddle.uk/?rdbms=oracle_18&fiddle=74371314913565243863c225847eb044

Answer 3

You can try the following query.
SELECT distinct
STUD.STUDENT_NAME,
STUD.STUDENT_ID
FROM STUDENT STUD
INNER JOIN CLASS CLS ON CLS.STUDENT_ID = STUD.STUDENT_ID
INNER JOIN LECTURE LEC ON LEC.LECTURE_ID=CLS.LECTURE_ID
where LEC.ECTS > 4 group by STUD.STUDENT_ID,STUD.STUDENT_NAME
having COUNT(STUD.STUDENT_ID) =(SELECT COUNT(*) FROM LECTURE WHERE ECTS > 4)