连接后在 groupconcat 中按日期分组

Question

我在 store 表中有一个商店列表，我还有一个 storecomment 表，其中包含每个商店的多个 cmets。它们通过 storeID 外键链接。

这是一个更大查询的子查询，用于将多条信息返回到按每个商店排序的单行中。此子查询的目的是将该商店的所有 cmets 返回到每个商店的单个列中，格式为 yyyy-mm-dd - COMMENT - Name 并在顶部按最新评论排序。我可以让字符串正常工作，但它总是无法订购。

LEFT JOIN (SELECT group_concat(storecomment.CommentDate, ' - ', storecomment.Comment, ' - ', users.Name, '\n' SEPARATOR '') as storecomments, StoreID from storecomment
                       inner join users on storecomment.CommentUserID = users.UserID 
                       ORDER BY CommentDate DESC
                       group by StoreID) 
                       as storecomments on store.StoreID = storecomments.StoreID

这主要是可行的，但是 cmets 的排序失败并且它按照输入的顺序出现。

我也尝试过按连接字符串的第一部分排序，例如：

LEFT JOIN (SELECT group_concat(storecomment.CommentDate, ' - ', storecomment.Comment, ' - ', users.Name, '\n' SEPARATOR '') as storecomments, StoreID from storecomment
                       inner join users on storecomment.CommentUserID = users.UserID
                       group by StoreID
                       Order by UNIX_TIMESTAMP(SUBSTRING(storecomments,9)) DESC)
                       as storecomments on store.StoreID = storecomments.StoreID

最后我尝试将日期时间转换为 unix 时间戳并以这种方式排序，但仍然无法排序：

LEFT JOIN (SELECT group_concat(storecomment.CommentDate, ' - ', storecomment.Comment, ' - ', users.Name, '\n' SEPARATOR '') as storecomments, StoreID from storecomment
                        inner join users on storecomment.CommentUserID = users.UserID
                        group by StoreID
                        Order by UNIX_TIMESTAMP(STR_TO_DATE(storecomment.CommentDate, '%Y-%m-%d %h:%i%p')) DESC
                        as storecomments on store.StoreID = storecomments.StoreID

我确信有一种简单的方法可以解决这个问题，但我就是看不到。有没有人有什么建议？

Answer 1

您可以订购 group_concat 内的值连接方式，因为它内置了对 order by 的支持。

将您的 group_concat 更改为如下所示：

group_concat(storecomment.CommentDate, ' - ', storecomment.Comment, ' - ', users.Name, '\n' ORDER BY storecomment.CommentDate DESC SEPARATOR '')

示例：

mysql> create table example (user_id integer, group_id integer);
Query OK, 0 rows affected (0.10 sec)

mysql> insert into example values (1, 1), (1, 2), (1, 3), (2, 7), (2, 4), (2, 5);
Query OK, 6 rows affected (0.07 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> select group_concat(group_id) from example group by user_id;
+------------------------+
| group_concat(group_id) |
+------------------------+
| 1,2,3                  |
| 7,4,5                  |
+------------------------+
2 rows in set (0.00 sec)

mysql> select group_concat(group_id order by group_id asc separator '-') from example group by user_id;
+------------------------------------------------------------+
| group_concat(group_id order by group_id asc separator '-') |
+------------------------------------------------------------+
| 1-2-3                                                      |
| 4-5-7                                                      |
+------------------------------------------------------------+
2 rows in set (0.00 sec)