The difference between countDistinct and distinct.count

Question

Why do I get different outputs for ..agg(countDistinct(\"member_id\") as \"count\") and ..distinct.count? Is the difference the same as between

Answer 1

Why do I get different outputs for ..agg(countDistinct("member_id") as "count") and ..distinct.count?

Because .distinct.count is the same:

SELECT COUNT(*) FROM (SELECT DISTINCT member_id FROM table)

while ..agg(countDistinct("member_id") as "count") is

SELECT COUNT(DISTINCT member_id) FROM table

and COUNT(*) uses different rules than COUNT(column) when nulls are encountered.