sql - 如何从映射表组中获取计数是 PostgreSQL 中的另一个表？

Question

我有三张桌子：

store 
=====
   name
   address 
   city 
   state 
   country 
   tag ..., 

post
=======
    title
    summary 
    tags ...

store_post_map
================
(mapping on store and post based on tag).

现在我想从映射表组中获取帖子计数city，或者state，PostgreSQL中的SQL是什么？countrystore.id

Answer 1

基本上是这样的：

SELECT store_id, count(*) AS posts_ct
FROM   store_post_map
GROUP  BY store_id;

我们如何获得每个地区可以有多个商店的每个城市、州或国家的计数？

每个国家的计数：

SELECT s.country, count(*) AS posts_ct
FROM   store          s
JOIN   store_post_map sp ON sp.store_id = s.id
GROUP  BY 1; -- positional parameter - is the same as GROUP BY s.country here

对于每个city您可能需要的计数GROUP BY area，country此外，因为城市名称几乎不是唯一的。像：

SELECT s.city, s.area, s.country, count(*) AS posts_ct
FROM   store          s
JOIN   store_post_map sp ON sp.store_id = s.id
GROUP  BY 1, 2, 3;