sql - 在 JOIN 查询中使用限制/偏移量

Question

我有4张 table

1. 用户帐户

   user_id | username | password                           
   ---------+----------+----------
   

2. 项目表

    project_id |         project_name         | category_id 
    ------------+------------------------------+-------------
   

3. user_projects 表(多对多关系)

   accounts_projects_id | account_id | project_id 
   ----------------------+------------+------------
   

4. project_messages 表(一个项目将有许多消息)

   message_id | project_id |message| username 
   ------------+------------+--------+---------
   

登录时，我运行一个查询，使用以下查询获取用户所属的项目数量以及每个项目的消息

SELECT account.user_id,account.username,
       array_agg(json_build_object('message',project_messages.message,'username',project_messages.username)) AS messages,
project.project_name 
FROM account 
  JOIN accounts_projects ON account.user_id  = accounts_projects.account_id
  JOIN project_messages ON accounts_projects.project_id = project_messages.project_id
  JOIN project ON project.project_id = accounts_projects.project_id
WHERE account.username=$1 
GROUP BY project.project_name,account.user_id

这给了我以下输出

userid,username, messages (json array object),project_name`
87;"kannaj";"{"{\"message\" : \"saklep\", \"username\" : \"kannaj\"}"}";"Football with Javascript"
87;"kannaj";"{"{\"message\" : \"work\", \"username\" : \"kannaj\"}","{\"message\" : \"you've been down to long in the midnight sea\", \"username\" : \"kannaj\"}","{\"message\" : \"Yeaaaa\", \"username\" : \"house\"}"}";"Machine Learning with Python"
87;"kannaj";"{"{\"message\" : \"holyy DIVVEERRR\", \"username\" : \"kannaj\"}"}";"Beethoven with react"

从project_messages表中检索消息时，有没有办法使用LIMIT/OFFSET函数？

Answer 1

为了让我们的示例更简单，假设我们有两个链接表:

t1(id);
t2(id, t1_id);

查询是

select t1.id, array_agg(t2.id)
from t1 join t2 on (t1.id = t2.t1_id)
group by t1.id;

如您所见，这是大型查询的非常简化的变体。

1) 数组

select t1.id, (array_agg(t2.id order by t2.id desc))[3:5]
from t1 join t2 on (t1.id = t2.t1_id)
group by t1.id;

此查询的工作方式与原始查询相同，但仅从数组的 3,4 和 5 个元素返回，这些元素等于offset 2 limit 3。

2) 子查询和横向

select
  t1.id,
  array_agg(t.x)
from
  t1 join lateral 
    (select t2.id as x from t2 where t1.id = t2.t1_id order by t2.id desc offset 2 limit 3) t on (true)
group by t1.id;

这里later关键字允许使用子查询(t1.id)中主from子句中提到的其他表中的字段。