SQL 使用 GROUP_CONCAT() 函数查询

Question

我有两个表：

1.Activity-- which stores all the activities from a user
2.User-- which stores all the registered users

我想在一个查询中获取所有 post 和所有 post 的类似内容。一行是 post 还是类似由 type 列的值决定。

Table 姓名 - Activity

|  actId  |  parentId  |  type     |  postedby    |
---------------------------------------------------
|  100    |  NULL      |  post     | 100          |
|  200    |  100       |  like     | 200          |
|  300    |  100       |  like     | 300          |
|  400    |  100       |  like     | 400          |
|  500    |  NULL      |  post     | 100          |
|  600    |  500       |  like     | 600          |

Table 姓名 - 用户

|  userId  |  name    |
-------------------
|  100     |  Amit    |
|  200     |  Alok    |
|  300     |  Arjun   |
|  400     |  Arpeet  |
|  600     |  Amaan   |

输出应该是

|  actId  |  name  |  LikedBy            |
------------------------------------------
|  100    |  Amit  |  Alok, Arjun, Arpeet|
|  500    |  Amit  |  Amaan              |

NOTE - 我不希望使用 FOR XML PATH 来实现这一点，因为它不受支持我正在使用的 SQLite。

我的查询尝试是

SELECT  a.id, u.name,
(select group_concat(u.name) from activity a, user u where a.id = u.id and a.type = 'like'
group by a.parent) as likedby
FROM activity a, user u
WHERE a.id = u.id  and a.type = 'post'

我得到的结果是

|  Id  |  name  |  LikedBy            |
---------------------------------------
|  100 |  Amit  |  Alok, Arjun, Arpeet|

您可以看到结果中缺少另一行。

Answer 1

试试这个：

select t.id4id Id, n1.Name Name, group_concat(n2.name) Groups
from (select t1.actid id4id, t1.postedby id4name, t2.postedby id4groups
        from Activity t1 join Activity t2 on t1.actid = t2.parentid) t join "user" n1 on t.id4name = n1.userid join "User" n2 on t.id4groups = n2.userid
group by id4id

注意事项： 1. 我假设 post 在 parent

中总是有一个空值

Answer 2

您不需要子查询。您只需要两个连接即可获取原始名称和喜欢的名称的值：

select a.parentid, u.name, group_concat(ul.name separator ', ') as likedby
from activity a join
     user ul
     on a.postedby = ul.userid join
     user u
     on a.parentid = u.userid
where a.type = 'like'
group by a.parentid, u.name;

SQL 使用 GROUP_CONCAT() 函数查询

SQL query using GROUP_CONCAT() function

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