左外连接先聚合

Question

我有以下表格

CREATE TABLE categories(
  id SERIAL,
);

CREATE TABLE category_translations(
  id SERIAL,
  name varchar not null,
  locale varchar not null, 
  category_id integer not null
);

CREATE TABLE products(
  id SERIAL,
  category_id integer not null
);

CREATE TABLE line_items(
  id SERIAL,
  total_cents integer
  product_id integer not null
);

我要做的是将每个类别名称的映射输出到与其关联的line_itemstotal_cents 的总和。比如：

name	sum_total_cents
Fresh foods	100000
Dry products	532000

有一个唯一性约束，即每个语言环境只存储一个名称。因此，category 将在 category_translations 表中存储的每个区域设置一行

我目前拥有的是

SELECT SUM(line_items.total_cents) AS sum_total_cents, ???
FROM line_items INNER JOIN products ON products.id = line_items.product_id
INNER JOIN categories ON categories.id = products.category_id
LEFT OUTER JOIN category_translations ON category_translations.category_id = categories.id 
WHERE category_translations.locale ='en'
GROUP BY categories.id

我正在寻找一个聚合函数来返回该类别的第一个 name。唯一缺少的是要写什么而不是???，因为我遇到了很多must appear in the GROUP BY clause or be used in an aggregate function 错误。在伪代码中，我正在寻找可以使用的 PostgreSQL 中的 FIRST() 聚合方法

Answer 1

假设您想要一个来自任何语言环境的 随机 名称，您可以这样做：

select
  c.id,
  (select name from category_translations t 
   where t.category_id = c.id limit 1) as name,
  sum(i.total_cents) as sum_total_cents
from categories c
left join products p on p.category_id = c.id
left join line_items i on i.product_id = p.id
group by c.id, name

或者，如果您想要区域设置“en”的类别名称，那么您可以这样做：

select
  c.id,
  (select t.name from category_translations t 
   where t.category_id = c.id and t.locale ='en') as name,
  sum(i.total_cents) as sum_total_cents
from categories c
left join products p on p.category_id = c.id
left join line_items i on i.product_id = p.id
group by c.id, name