如果不满足条件，如何在 array_agg 中获取空数组？

Question

我有 2 个表：

collections_books (collection_id, book_id)
users_collections (user_id, collection_id, access)

我正在使用 PostgreSQL。

以下查询为我提供了按 collection_id 分组的 book_id 的列表。问题是，由于我使用的是 where 条件，结果仅限于集合 allowed for user_id = 3.

但是，我想要所有的 collection_ids 和相应的 book_ids

作为数组，如果 user_id 有 access = allow
作为空数组，如果 user_id 不存在于 users_collections 或 user_id != allow

SELECT c.collection_id, ARRAY_AGG(c.book_id)
FROM collections_books AS c
LEFT JOIN users_collections AS u
ON c.collection_id = u.collection_id
WHERE 
  u.access = 'allow' AND
  u.user_id = 3
GROUP BY c.collection_id;

Answer 1

您检查 array_length()。如果它小于 1 那么 return 一个值为 {null}

的数组

如果您想从 collections_books 中获取所有 collection_id，但仅当 u.access = 'allow' AND u.user_id 时获取 book_id 的数组= 4 否则 null 然后使用下面的查询：

SELECT c.collection_id, 
    (
        CASE 
             WHEN max(u.access) = 'allow' AND max(u.user_id) = 4
             THEN ARRAY_AGG(c.book_id)
             ELSE '{null}'::int[] 
        END
    )
    FROM collections_books AS c
    LEFT JOIN users_collections AS u
    ON c.collection_id = u.collection_id
    GROUP BY c.collection_id;

Answer 2

请查看以下答案，让我知道它是否 returns 您想要的输出：

架构和插入语句：

     create table users_collections (user_id int, collection_id int, access varchar(20));
     insert into users_collections values(3, 1, 'allow');
     insert into users_collections values(3, 2, 'allow');
     insert into users_collections values(4, 3, 'allow');
     insert into users_collections values(3, 5, 'not allow');

 
     create table collections_books (collection_id int, book_id int);
     insert into collections_books values(2,24);
     insert into collections_books values(3,35);
     insert into collections_books values(3,25);
     insert into collections_books values(1,36);
     insert into collections_books values(1,22);
     insert into collections_books values(1,24);
     insert into collections_books values(2,34);
     insert into collections_books values(5,344);
     insert into collections_books values(6,474);

查询：

     SELECT c.collection_id, (CASE WHEN max(u.access) = 'allow' AND max(u.user_id) = 3
     THEN ARRAY_AGG(c.book_id)
     ELSE '{null}'::int[] END)
     FROM collections_books AS c
     LEFT JOIN users_collections AS u
     ON c.collection_id = u.collection_id
     GROUP BY c.collection_id;

输出：

 |collection_id | case      |
 |------------: | :---------|
 |            3 | {35,25}   |
 |            5 | {NULL}    |
 |            6 | {NULL}    |
 |            2 | {24,34}   |
 |            1 | {36,24,22}|

dbhere

如果不满足条件，如何在 array_agg 中获取空数组？

How to get an empty array in array_agg if condition is not met?

sql

postgresql

join

array-agg