sql - GROUP BY 包括 0，其中不存在

Question

我有一个表lists，每个表都包含posts. posts我想要一个查询，告诉我每个有多少list，包括每个没有任何帖子的列表的 0 条目。

例如。

posts：

id  |  list_id
--------------
1   |   1  
2   |   1
3   |   2
4   |   2

lists：

id
---
1
2
3

应该返回：

list_id | num_posts
-------------------
1       | 2
2       | 2
3       | 0

我已经使用以下查询这样做了，但是有效地进行分组然后执行另一个子查询来填补空白感觉有点愚蠢：

WITH "count_data" AS (
  SELECT "posts"."list_id" AS "list_id", COUNT(DISTINCT "posts"."id") AS "num_posts"
  FROM "posts"
  INNER JOIN "lists" ON "posts"."list_id" = "lists"."id"
  GROUP BY "posts"."list_id"
)
SELECT "lists"."id", COALESCE("count_data"."num_posts", 0)
FROM "lists"
LEFT JOIN "count_data" ON "count_data"."list_id" = "lists"."id"
ORDER BY "count_data"."num_posts" DESC

谢谢！

Answer 1

直接左连接会更有效，避免在此过程中使用大合并连接进行 seq 扫描：

select lists.id as list_id, count(posts.list_id) as num_posts
from lists
left join posts on posts.list_id = lists.id
group by lists.id

Answer 2

If I understand your question, this should work:

SELECT List_ID, ISNULL(b.list_ID,0)
FROM lists a
LEFT JOIN (SELECT list_ID, COUNT(*)
      FROM posts
      GROUP BY list_ID
     )b
ON a.ID = b.list_ID