sql - 限制嵌套 JOIN 查询的结果

Question

老实说，我不知道如何给这个更好的标题 :(

基本上我有这3张表

Table "public.users"
Column |         Type          |                     Modifiers
--------+-----------------------+----------------------------------------------------
id     | integer               | not null default nextval('users_id_seq'::regclass)
name   | character varying(40) |
Indexes:
"users_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "comments" CONSTRAINT "comments_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON UPDATE CASCADE ON DELETE CASCADE

Table "public.comments"
Column  |  Type   |                       Modifiers
---------+---------+-------------------------------------------------------
id      | integer | not null default nextval('comments_id_seq'::regclass)
user_id | integer |
content | text    |
Indexes:
"comments_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"comments_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON UPDATE CASCADE ON DELETE CASCADE

Table "public.votes"
Column   |  Type   |                     Modifiers
------------+---------+----------------------------------------------------
id         | integer | not null default nextval('votes_id_seq'::regclass)
up         | boolean | default false
comment_id | integer |
Indexes:
"votes_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"votes_comment_id_fkey" FOREIGN KEY (comment_id) REFERENCES comments(id) ON UPDATE CASCADE ON DELETE CASCADE

我想选择所有用户(包括那些没有任何评论的用户)并为每个用户包含 3 条评论，然后为每个评论选择 2 票(包括那些没有任何投票的评论)

到目前为止，我的查询是为每个用户选择 3 条评论

SELECT users.id as userId, comments.id as commentId, users.name, comments.content, comments.rn
FROM users
LEFT JOIN (
  SELECT *, row_number() OVER (PARTITION BY comments.user_id) as rn FROM comments
) as comments
ON users.id = comments.user_id
WHERE comments.rn <= 3 OR comments.rn IS NULL;

Answer 1

你的想法是正确的。继续吧:

SELECT u.id as userId, c.id as commentId, u.name, c.content, c.rn
FROM users u LEFT JOIN
     (SELECT c.*,
             ROW_NUMBER() OVER (PARTITION BY c.user_id) as rn
      FROM comments c
     ) c
    ON u.id = c.user_id LEFT JOIN
    (SELECT v.*,
            ROW_NUMBER() OVER (PARTITION BY v.comment_id) as rn
     FROM votes v
    ) v
    ON c.id = v.comment_id
WHERE (c.rn <= 3 OR c.rn IS NULL) and
      (v.rn <= 2 or v.rn IS NULL);