Count Distinct 小于 Sum(Count Distinct)

Question

我有两个问题：

select COUNT(DISTINCT (CASE WHEN EVENT_NAME = 'event' THEN UPPER(user END)) AS SIGNUP_COUNT,
from table
WHERE date BETWEEN '2020-07-01' AND '2020-09-01'

和

with EVENTS_FILTERED_with_count as (  
select *
, COUNT(DISTINCT (CASE WHEN EVENT_NAME = 'event' THEN UPPER(user END)) AS SIGNUP_COUNT
from table
group by 1)

SELECT sum(SIGNUP_COUNT) FROM EVENTS_FILTERED_with_count
WHERE date BETWEEN '2020-07-01' AND '2020-09-01'

第一个查询 returns 2.5K 行作为结果，第二个查询 returns 3K 行。

为什么加上group by会使结果变大？我想知道它是否与 NULL 值有关。

Answer 1

因为同一个user有多个事件，所以在用户级统计时，该事件会被多次统计

没有样本数据很难描述得更清楚。

Count Distinct 小于 Sum(Count Distinct)

Count Distinct is less than Sum(Count Distinct)

sql

distinct