计算组中的不同行

Question

我有一个表格列表＆＃34;动作＆＃34;，其中每个动作都有一个时间和执行它的人。我喜欢的是知道每天执行任何行动（并不重要）的不同人的数量。所以我的表看起来像：

action_time         | person | action
--------------------+--------+-------
2017-05-10 10:00:00 | john   | logged on
2017-05-10 10:30:00 | paul   | logged on
2017-05-10 17:50:00 | john   | logged off
2017-05-10 18:00:00 | paul   | logged off
2017-05-11 12:00:00 | john   | read email
2017-05-12 12:00:00 | john   | read email
2017-05-13 10:00:00 | john   | read email
2017-05-13 10:30:00 | paul   | read email
2017-05-13 10:45:00 | george | read email

我希望能够得到，结果

day        | count of people doing things
-----------+-----------------------------
2017-05-10 | 2 (note: john and paul each did 2 different things; don't care)
2017-05-11 | 1 (note: john only)
2017-05-12 | 1 (note: john only)
2017-05-13 | 3 (note: john, paul, and george)

显然我可以GROUP BY DATE(action_time)（或某些特定于数据库的版本; sqlite中的group by strftime('%Y-%m-%d', action_time)等），我也可以按person进行分组......但是{{1为第一天做事的人数提供 4 ...即，它计算行数，而不是人数 （这是2，而不是4）。我不认为我正确理解select date(action_time), count(person) from actions group by date(action_time)。我该怎么办？

Answer 1

SELECT DATE(action_time), COUNT(DISTINCT person) FROM ACTIONS GROUP BY DATE(action_time);