右表行不存在时的PostgreSQL横向连接

Question

当我的数据库中有一个没有帖子的类别时，结果选择返回没有空类别的行。如何接收一个空类别的表格。

  SELECT
    c.id as category_id,
    pp.id as post_id
  FROM categories c,
  LATERAL
  (SELECT
    id
  FROM posts
  WHERE c.id = posts.category_id
   ORDER BY views DESC
   LIMIT 2) pp
  ORDER BY c.category ASC, pp.id ASC

当前查询结果：

category_id | post_id
----------------------
1           | 1
1           | 2
3           | 3
3           | 4

我需要：

category_id | post_id
----------------------
1           | 1
1           | 2
2           |
3           | 3
3           | 4

Answer 1

使用left join:

SELECT
    c.id as category_id,
    pp.id as post_id
FROM categories c
LEFT JOIN LATERAL
(SELECT
    id
FROM posts
WHERE c.id = posts.category_id
    ORDER BY views DESC
    LIMIT 2) pp ON TRUE
ORDER BY c.category ASC, pp.id ASC

请注意，我使用on true 作为连接条件，因为横向连接引用了上一个表。