如何使用查询 array_agg 制作此 sql？

Question

我要查询

select * from projects where user_id = 3;

并根据结果 r，我需要进行 n 查询，其中 n 是长度 l 的 r。例如：

| id | project_name | description | user_id |
| 1  | Project A    | lorem ipsu  | 3       |
| 4  | Project B    | lorem ipsu  | 3       |

l => 2 然后:

select * from images where project_id = 1;
select * from images where project_id = 4;

好的，如果 l 太大了，你可以看到这是怎么回事。选择太多，对数据库的访问太多。有没有更好的方法来达到这样的最终结果：

| id | project_name | description | user_id | images           |
| 1  | Project A    | lorem ipsu  | 3       | {imgX,imgY,imgZ} |
| 4  | Project B    | lorem ipsu  | 3       | {imgA,imgB}      |

我听说了 postgres 上的 array_agg 函数。也许这就是答案？无论如何，这些是我的 table 描述：

                             Table "public.projects"
   Column    |           Type           |                           Modifiers                       
-------------+--------------------------+-------------------------------------------------------
 id          | integer                  | not null default     nextval('projects_id_seq'::regclass)
 name        | character varying(255)   | 
 description | character varying(255)   | 
 user_id     | integer                  | 
 created_at  | timestamp with time zone | 
 updated_at  | timestamp with time zone | 

                                    Table "public.images"
   Column   |           Type           |                      Modifiers                      
------------+--------------------------+-----------------------------------------------------
 id         | integer                  | not null default nextval('images_id_seq'::regclass)
 name       | character varying(255)   | 
 url        | character varying(255)   | 
 project_id | integer                  | 
 created_at | timestamp with time zone | 
 updated_at | timestamp with time zone |

提前谢谢你:D

Answer 1

可能对您来说最简单的解决方案是子select。这最接近您之前提到的个别 SELECT 陈述：

SELECT * FROM images
WHERE project_id IN (
  SELECT project_id FROM projects
  WHERE user_id = 3);

Answer 2

您想要 projects 中的记录加上 image names as array 中匹配 project_id 的记录：

SELECT *
FROM   projects
LEFT JOIN LATERAL (SELECT array_agg(name) AS images FROM images WHERE project_id = projects.project_id) x ON true
WHERE user_id = '3'

sqlfiddle

Answer 3

array_agg 类似于任何其他聚合函数（计数、求和），但 returns 是一个数组而不是标量值。只需连接和分组 2 个表即可实现您的需要。

SELECT p.id, p.name, p.description, p.user_id, array_agg(i.name) images
FROM projects p
LEFT JOIN images i ON p.id = i.project_id
GROUP BY p.id, p.name, p.description, p.user_id

如何使用查询 array_agg 制作此 sql？

How to make this sql with query array_agg?

sql

arrays

postgresql

join

array-agg