SUM(CASE WHEN ...) returns a greater number than COUNT(DISTINCT..) - postgresql

Question

I have written a query in two models, but I can't figure out why the second query returns a greater number than the first one; while the number that the first one, COUNT(DISTINCT...) returns is correct:
WITH types(id) AS (VALUES('{1, 4, 5, 3}'::INTEGER[])),
date_gen64 AS
(
SELECT CAST (generate_series(date '10/1/2017', date '11/15/2017', interval
'1 day') AS date) as days ORDER BY days)
SELECT cl.class_date AS c_date,
count(DISTINCT (CASE WHEN co.id = 1 THEN p.id END)),
count(DISTINCT (CASE WHEN co.id = 2 THEN p.id END))
FROM person p
JOIN envelope e ON e.personID = p.id
JOIN "class" cl on cl.id = p.classID
JOIN course co ON co.id = cl.course_id AND co.id = 1
JOIN types ON cr.type_id = ANY (types.id)
RIGHT JOIN date_gen64 dg ON dg.days = cl.class_date
GROUP BY cl.class_date
ORDER BY cl.class_date
The above query returns 26 but following query returns 27!
The reason why I rewrote it with SUM is that the first query
was too slow. But my question is that why the second one counts more?
WITH types(id) AS (VALUES('{1, 4, 5, 3}'::INTEGER[]))
SELECT tmpcl.days,
SUM(CASE WHEN tmp80.course_id = 1 THEN 1
ELSE 0 END),
SUM(CASE WHEN tmp80.course_id = 2 THEN 1
ELSE 0 END)
FROM (
SELECT CAST (generate_series(date '10/1/2017', date '11/15/2017',
interval '1 day') AS date) as days ORDER BY days) tmpcl
LEFT JOIN (
SELECT DISTINCT p.id AS "person_id",
cl.class_date AS c_date,
co.id AS "course_id"
FROM person p
JOIN envelope e ON e.personID = p.id
JOIN "class" cl on cl.id = p.classID
JOIN course co ON co.id = cl.course_id
JOIN types ON cr.type_id = ANY (types.id)
WHERE co.id IN ( 1 , 2 )
) tmp80 ON tmpcl.days = tmp80.class_date
GROUP BY tmpcl.days
ORDER BY tmpcl.days

Answer 1

You can theoretically have multiple people enrolled in the same class on the same day. Indeed that would seem to be the main point of having classes. So each time there are multiple people assigned to the same class on the same day you can have a higher count than you would in your first query. Does that make sense?
You don't appear to be using p.id in that inner query so simply remove it and your counts should match.
WITH types(id) AS (VALUES('{1, 4, 5, 3}'::INTEGER[]))
SELECT tmpcl.days,
SUM(CASE WHEN tmp80.course_id = 1 THEN 1
ELSE 0 END),
SUM(CASE WHEN tmp80.course_id = 2 THEN 1
ELSE 0 END)
FROM (
SELECT CAST (generate_series(date '10/1/2017', date '11/15/2017',
interval '1 day') AS date) as days ORDER BY days) tmpcl
LEFT JOIN (
SELECT DISTINCT cl.class_date AS c_date,
co.id AS "course_id"
FROM person p
JOIN envelope e ON e.personID = p.id
JOIN "class" cl on cl.id = p.classID
JOIN course co ON co.id = cl.course_id
JOIN types ON cr.type_id = ANY (types.id)
WHERE co.id IN ( 1 , 2 )
) tmp80 ON tmpcl.days = tmp80.class_date
GROUP BY tmpcl.days
ORDER BY tmpcl.days