Need SQL query to export students attendance report by classes - sql

Question

I have 3 tables:
students table
id
name
1
Jhon
2
Emma
3
Oliver
classes table
id
name
1
Math
2
Science
attendance table
id
student_id
class_id
1
1
1
2
1
2
3
2
1
I tried to write an SQL query to retrieve the students who attended both math and science classes:
SELECT s.id, s.name
FROM attendance a
INNER JOIN students s ON a.student_id = s.id
WHERE a.class_id IN (1,2);
the above code result is
id
name
1
Jhon
2
Emma
1
Jhon
But Emma only attended Math class.
I know this behavior is because of WHERE IN, it's the same as WHERE a.class_id = 1 OR a.class_id = 2.
But what should I do to get the expected results, Which is "Jhon" or only Jhon's ID because he is the only one who attended Math and Science Classes?

Answer 1

Either join to attendance twice, once for each class:
SELECT s.id, s.name
FROM students s
JOIN attendance a1 ON a1.student_id = s.id and a1.class_id = 1
JOIN attendance a2 ON a2.student_id = s.id and a2.class_id = 2
or join to both classes at once and use group by with having:
SELECT s.id, s.name
FROM students s
JOIN attendance a ON a.student_id = s.id and class_id in (1, 2)
GROUP BY s.id, s.name
HAVING COUNT(*) = 2

Answer 2

Using a CTE would help, and can help you extend to multiple classes. You are also missing a DISTINCT keyword, which is why Jhon appears twice. Maybe something like this:
WITH attendance_count(student_id, classes_attended)
AS (
SELECT
student_id,
COUNT(id) AS classes_attended
FROM attendance a
-- change this to change the classes
WHERE a.class_id IN (1,2)
GROUP BY student_id
)
SELECT DISTINCT s.id, s.name
FROM attendance_count
INNER JOIN students s ON attendance_count.student_id = s.id
-- if different number of classes, change this
WHERE classes_attended = 2