如何从记录中提取值作为postgresql中的单个列

Question

如何从记录中提取值作为postgresql中的单个comuns

SELECT 
p.*,
(SELECT ROW(id,server_id,format,product_id) FROM products_images pi WHERE pi.product_id = p.id LIMIT 1) AS image

FROM products p

WHERE p.company = 1 ORDER BY id ASC LIMIT 10

代替

image 
(3, 4, "jpeg", 7)

我想拥有

id | server_id | format | product_id
3  | 4         | jpeg   | 7

有没有办法为每个产品只选择一个图像并直接返回列而不是记录？

Answer 1

试试这个:

create type xxx as (t varchar, y varchar, z int);

with a as
(
select row(table_name, column_name, (random() * 100)::int) x 
from information_schema.columns
)
-- cannot cast directly to xxx, should cast to text first
select (x::text::xxx).t, (x::text::xxx).y, (x::text::xxx).z
from a

或者,您可以这样做:

with a as
(
select row(table_name, column_name, (random() * 100)::int) x 
from information_schema.columns
), 
-- cannot cast directly to xxx, should cast to text first
b as (select x::text::xxx as w from a)

select 
(w).t, (w).y, (w).z
from b

要选择所有字段:

with a as
(
select row(table_name, column_name, (random() * 100)::int) x 
from information_schema.columns
), 
-- cannot cast directly to xxx, should cast to text first
b as (select x::text::xxx as w from a)

select
(w).*
from b

你也可以这样做,但是当你可以删除ROW函数并从cte/derived表外部重新拾取它时,这使得使用ROW的整个练习变得毫无意义.我猜测OP的ROW来自一个功能; 他应该使用上面的代码,而不是以下代码:

with a as
(
select row(table_name, column_name, (random() * 100)::int)::xxx x 
from information_schema.columns
)
select 
(x).t, (x).y, (x).z
from a

Answer 2

只需指定结构的组件：

SELECT a,b,c,(image).id, (image).server_id, ...
FROM (

SELECT 
p.*,
(SELECT ROW(id,server_id,format,product_id) FROM products_images pi WHERE pi.product_id = p.id LIMIT 1) AS image

FROM products p

WHERE p.company = 1 ORDER BY id ASC LIMIT 10
) as subquery

但无论如何，我会重新设计查询并使用联接而不是子条款。

 SELECT DISTINCT ON (p.*) p.*,
        p.id,pi.server_id,pi.format,pi.product_id
   FROM products p
   LEFT JOIN product_images pi ON pi.product_id = p.id
  WHERE p.company = 1 
  ORDER BY id ASC 
  LIMIT 10

但我相信您必须分别指定不同的所有 p 字段，以确保每个产品仅加载一张图像。