Database 如何创建在Postgresql中返回动态列名的查询？

我在一个报告数据库中有两个表，一个用于订单，另一个用于订单项。每个订单可以有多个订单项目，以及每个项目的数量：

Orders
+----------+---------+
| order_id | email   |
+----------+---------+
| 1        | 1@1.com |
+----------+---------+
| 2        | 2@2.com |
+----------+---------+
| 3        | 3@3.com |
+----------+---------+

Order Items
+---------------+----------+----------+--------------+
| order_item_id | order_id | quantity | product_name |
+---------------+----------+----------+--------------+
| 1             | 1        | 1        | Tee Shirt    |
+---------------+----------+----------+--------------+
| 2             | 1        | 3        | Jeans        |
+---------------+----------+----------+--------------+
| 3             | 1        | 1        | Hat          |
+---------------+----------+----------+--------------+
| 4             | 2        | 2        | Tee Shirt    |
+---------------+----------+----------+--------------+
| 5             | 3        | 3        | Tee Shirt    |
+---------------+----------+----------+--------------+
| 6             | 3        | 1        | Jeans        |
+---------------+----------+----------+--------------+

出于报告目的，我希望将此数据反规范化为一个单独的PostgreSQL视图（或仅运行一个查询），该视图将上述数据转换为如下内容：

+----------+---------+-----------+-------+-----+
| order_id | email   | Tee Shirt | Jeans | Hat |
+----------+---------+-----------+-------+-----+
| 1        | 1@1.com | 1         | 3     | 1   |
+----------+---------+-----------+-------+-----+
| 2        | 2@2.com | 2         | 0     | 0   |
+----------+---------+-----------+-------+-----+
| 3        | 3@3.com | 3         | 1     | 0   |
+----------+---------+-----------+-------+-----+

CREATE EXTENSION tablefunc;

SELECT * FROM crosstab
(
  'SELECT orders_id, product_name, quantity FROM orders_items ORDER BY 1',
  'SELECT DISTINCT product_name FROM orders_items ORDER BY 1'
)
AS
(
       orders_id text,
       TShirt text,
       Jeans text,
       Hat text
);

---

也就是说，它是订单中每个项目的数量加上产品名称的总和；以及设置为列标题的产品名称。我是否需要使用交叉表之类的工具来实现这一点，或者即使在查询运行之前我不知道不同产品名称的列表，是否有一种聪明的方法来使用子查询。

这是一个可能的答案：

create table orders 
(
  orders_id int PRIMARY KEY, 
  email text NOT NULL
);

create table orders_items 
(
  order_item_id int PRIMARY KEY, 
  orders_id int REFERENCES orders(orders_id) NOT NULL, 
  quantity int NOT NULL, 
  product_name text NOT NULL
);

insert into orders VALUES (1, '1@1.com');
insert into orders VALUES (2, '2@2.com');
insert into orders VALUES (3, '3@3.com');

insert into orders_items VALUES (1,1,1,'T-Shirt');
insert into orders_items VALUES (2,1,3,'Jeans');
insert into orders_items VALUES (3,1,1,'Hat');
insert into orders_items VALUES (4,2,2,'T-Shirt');
insert into orders_items VALUES (5,3,3,'T-Shirt');
insert into orders_items VALUES (6,3,1,'Jeans');


select 
  orders.orders_id, 
  email,
  COALESCE(tshirt.quantity, 0) as "T-Shirts",
  COALESCE(jeans.quantity,0) as "Jeans",
  COALESCE(hat.quantity, 0) as "Hats"
from 
  orders 
  left join (select orders_id, quantity from orders_items where product_name = 'T-Shirt') 
    as tshirt ON (tshirt.orders_id = orders.orders_id)
  left join (select orders_id, quantity from orders_items where product_name = 'Jeans') 
    as jeans ON (jeans.orders_id = orders.orders_id)
  left join (select orders_id, quantity from orders_items where product_name = 'Hat') 
    as hat ON (hat.orders_id = orders.orders_id)
;

用postgresql测试。结果:

 orders_id |  email  | T-Shirts | Jeans | Hats 
-----------+---------+----------+-------+------
         1 | 1@1.com |        1 |     3 |    1
         2 | 2@2.com |        2 |     0 |    0
         3 | 3@3.com |        3 |     1 |    0
(3 rows)

根据您的评论，您可以尝试这样使用：

+----------+---------+-----------+-------+-----+
| order_id | email   | Tee Shirt | Jeans | Hat |
+----------+---------+-----------+-------+-----+
| 1        | 1@1.com | 1         | 3     | 1   |
+----------+---------+-----------+-------+-----+
| 2        | 2@2.com | 2         | 0     | 0   |
+----------+---------+-----------+-------+-----+
| 3        | 3@3.com | 3         | 1     | 0   |
+----------+---------+-----------+-------+-----+

CREATE EXTENSION tablefunc;

SELECT * FROM crosstab
(
  'SELECT orders_id, product_name, quantity FROM orders_items ORDER BY 1',
  'SELECT DISTINCT product_name FROM orders_items ORDER BY 1'
)
AS
(
       orders_id text,
       TShirt text,
       Jeans text,
       Hat text
);

---

但是我认为您对SQL的想法是错误的。您通常知道需要哪些行，并且必须告诉它SQL。“旋转桌子”90度不是SQL的一部分，应该避免

这很好，而且确实有效。但我真正想要的是这样一个查询，它可以扩展到30个产品名称，如果我不知道不同的产品名称列表，那么可以通过

从订单项中选择不同的（产品名称）这样的查询来检索产品名称，使用crosstab@Joe：你能解决问题吗？没有特别好的查询。不幸的是，我不得不在查询中列出我的每种产品。仍然在寻找使用交叉表的好方法吗