Question

我有一张lists表，每个表都包含posts。我想要一个查询，告诉我每个posts有多少list个，包括每个没有任何帖子的列表为0的条目。

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

我使用以下查询完成了此操作，但有效地进行分组然后执行另一个子查询以填充空白感觉有点愚蠢：

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

直接离开连接会更有效，避免在此过程中使用大型合并连接进行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

如果我理解你的问题，这应该有效：

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

GROUP BY包括0，其中不存在

2 个答案: