Question

老实说，我不知道如何为此提供更好的标题:(

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（包括那些没有评论的用户）并为每个用户添加3条评论，然后为每条评论选择2票（包括那些没有任何投票的评论）

到目前为止，我为每个用户选择3条评论的查询

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

因为对投票的评论和评论的用户都具有1对多的关系，所以两个联接都会为每个用户提供CxV（3x2 = 6）行。

由于评论的投票次数超过1次时评论会重复，因此请使用dense_rank()代替row_number()，以便“重复”评论具有相同的排名并包含在{{1}中}标准

rn < 4

Answer 2

你有正确的想法。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);

Answer 3

可能它更适合索引：

select
  *
from
  users u
    left join lateral (select * from comments c where u.id = c.user_id order by c.id limit 3) c on (u.id = c.user_id)
      left join lateral (select * from votes v where c.id = v.comment_id order by v.id limit 2) v on (c.id = v.comment_id)

限制嵌套JOIN查询的结果

3 个答案: