如何進行查詢？

Question

讓我們想象一下，我們有表的任務包含

user_id date 
1   2015-01-01  
1   2015-01-01  
1   2015-01-02   
2   2015-01-03  

如何像下面

user_id  count  date 
    1   2   2015-01-01 
    1   1   2015-01-02 
    1   0   2015-01-03 

    2   0   2015-01-01 
    2   0   2015-01-02 
    2   1   2015-01-03  

Answer 1

如果零是重要，那麼你可能會做這樣的事情顯示數據：

select 
    user_id_date_crossjoin.user_id, 
    user_id_date_crossjoin.`date`, 
    coalesce(`count`, 0) as `count` 
from 
    (select 
     user_id, 
     `date` 
    from 
     (select distinct user_id from tasks) all_userids, 
     (select distinct `date` from tasks) all_userid_dates 
    ) user_id_date_crossjoin 
left outer join 
    (select 
     user_id, 
     count(*) as `count`, 
     `date` 
    from tasks 
    group by user_id, `date`) user_id_date_counts 
on user_id_date_crossjoin.user_id = user_id_date_counts.user_id 
and user_id_date_crossjoin.`date` = user_id_date_counts.`date` 

請參閱http://sqlfiddle.com/#!9/e17090/3