MySQL count（）没有返回正确的数字

Question

This is the query I'm trying.

select
    UserId,
    count(e.UserId) as 'Experiments',
    count(t.TaskId) as 'Tasks tried',
    count(case when t.TaskStatus = 'Completed' then t.TaskStatus end) as 'Tasks completed',
    sec_to_time(avg(timediff(TaskLocalUserEndDateTime,TaskLocalUserStartDateTime))) as 'Average Time'
from
    Tasks as t,
    Experiments as e
where
    t.ExperimentId = e.ExperimentId
and
    e.UserId = e.UserId
group by
    UserId;

count(e.UserId) gives the same result as count(t.TaskId) which is wrong. But count() returns the correct results if I run the query without any joins, for example:

select
    count(UserId)
from
    Experiments
where
    UserId = UserId
group by 
    UserId;

Answer 1

Not sure this will help but do you get desired result if you run below query - see group by:

select
    count(UserId), ExperimentId
from
    Experiments
where
    UserId = UserId
group by 
    ExperimentId;

Just checking the query - noticed join between Tasks and Experiments tables is on ExperimentId - where in the query that you state you get the desired result only userId is used. Let me know if this helps.

select
    UserId,
    e.Experiments, -- count(e.UserId) as 'Experiments',
    count(t.TaskId) as 'Tasks tried',
    count(case when t.TaskStatus = 'Completed' then t.TaskStatus end) as 'Tasks completed',
    sec_to_time(avg(timediff(TaskLocalUserEndDateTime,TaskLocalUserStartDateTime))) as 'Average Time'
from
    Tasks as t,
--    Experiments as e 
LEFT JOIN 
(
   select count(UserId) as 'Experiments', ExperimentId from Experiments group by ExperimentId
) as e
where
    t.ExperimentId = e.ExperimentId
and
    e.UserId = e.UserId
group by
    UserId;