GROUP_CONCAT选项中的条件

Question

我有3个表格，我正在加入他们以获取一些数据。

 
----------------- 
Table Name: users 
------------------------------- 
|user_id | user_name | 
------------------------------- 
123  | abc 
------------------------------- 
223  | bcd 
------------------------------- 
323  | cde 
------------------------------- 

----------------- 
Table Name: limit 
------------------------------- 
user_id | limit_id 
------------------------------- 
123  | 1 
------------------------------- 
223  | 2 
------------------------------- 
323  | 3 
------------------------------- 
323  | 4 
------------------------------- 

------------------------- 
Table Name: limit_setting 
------------------------------- 
limit_id | date_limit 
------------------------------- 
1  | 2016-09-29 12:00:00 
------------------------------- 
2  | 2016-09-28 12:00:00 
------------------------------- 
3  | 2016-09-27 12:00:00 
------------------------------- 
1  | 2016-09-27 12:00:00 
------------------------------- 
1  | 2016-09-24 12:00:00 
------------------------------- 
4  | 2016-09-25 12:00:00 
------------------------------- 
4  | 2016-09-26 12:00:00 
------------------------------- 

我需要得到这样的结果。我被日期栏的GROUP_CONCAT卡住了。 日期列应包含除MAX日期以外的所有条目。如果在limit_id的limit_setting表中只有一个条目，那么它不应该为该用户显示任何内容。 count_dates：它在limit_setting表中存在的条目数。

 

Desired output 

---------------------------------------------------------------------- 
user_name | dates          | count_dates 
---------------------------------------------------------------------- 
abc  | 2016-09-27 12:00:00 , 2016-09-24 12:00:00 | 3 
---------------------------------------------------------------------- 
bcd  |            | 1 
---------------------------------------------------------------------- 
cde  |            | 1 
----------------------------------------------------------------------- 
cde  | 2016-09-26 12:00:00       | 2 
----------------------------------------------------------------------- 

SELECT PP.`user_name`, count(ESL.Limit_id) as count_dates, 
    GROUP_CONCAT(ESL.date_limit SEPARATOR ',') as dates 
FROM users as PP INNER JOIN `limit` as PAL ON PP.Id = PAL.PlayerId 
LEFT JOIN limit_setting as ESL ON ESL.LimitId = PAL.limitId 
GROUP BY PAL.limitId 

此外，我试着用（这什么也没有返回）

SELECT ESL.date_limit, MAX(date_limit) as max_date, PP.`user_name`, count(ESL.Limit_id) as count_dates, 
    GROUP_CONCAT(ESL.date_limit SEPARATOR ',') as dates 
FROM users as PP INNER JOIN `limit` as PAL ON PP.Id = PAL.PlayerId 
LEFT JOIN limit_setting as ESL ON ESL.LimitId = PAL.limitId 
GROUP BY PAL.limitId 
HAVING ESL.date_limit > max_date 

我试着用Find_in_set但不知道如何有效地使用它。

Answer 1

试试这个：

SELECT user_name,   
     CASE WHEN COUNT(*) > 1 
      THEN SUBSTRING_INDEX(GROUP_CONCAT(date_limit ORDER BY date_limit), 
           ',', COUNT(*) - 1) 
      ELSE '' 
     END AS dates,    
     COUNT(*) AS count_dates 
FROM users as PP 
INNER JOIN `limit` as PAL ON PP.user_id = PAL.user_id 
LEFT JOIN limit_setting as ESL ON ESL.limit_id = PAL.limit_id 
GROUP BY user_name 

查询使用SUBSTRING_INDEX功能，以获得通过GROUP_CONCAT返回的所有日期除外的最后日期。在GROUP_CONCAT内使用ORDER BY，我们可以在最后放置最大日期，以便SUBSTRING_INDEX恰好截断此日期。

Demo here

Answer 2

我会用一个子查询返回按极限的最大日期和加入这一回限制设置表，以消除来自group_concat()最大日期与case表达：

SELECT 
    PP.`user_name`, 
    count(ESL.Limit_id) as count_dates, 
    GROUP_CONCAT(CASE 
        WHEN ESL.date_limit<>ls.maxdate THEN ESL.date_limit 
        ELSE '' 
       END SEPARATOR ',') as dates 
FROM users as PP 
INNER JOIN `limit` as PAL ON PP.Id = PAL.PlayerId 
LEFT JOIN limit_setting as ESL ON ESL.LimitId = PAL.limitId 
LEFT JOIN (SELECT LimitId, MAX(date_limit) as maxdate 
      FROM limit_setting 
      GROUP BY LimitId 
     ) ls ON ESL.LimitId=ls.LimitId 
GROUP BY PP.`user_name`