database – 如何在Postgresql中创建返回动态列名的查询？

发布时间：2020-12-13 16:07:33 所属栏目：百科来源：网络整理

导读：我在报告数据库中有两个表,一个用于订单,一个用于订单项.每个订单可以包含多个订单商品,以及每个订单商品的数量： Orders+----------+---------+| order_id | email |+----------+---------+| 1 | 1@1.com |+----------+---------+| 2 | 2@2.com |+---------

我在报告数据库中有两个表,一个用于订单,一个用于订单项.每个订单可以包含多个订单商品,以及每个订单商品的数量：

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,我喜欢将这些数据反规范化为一个单独的PostgreSQL视图(或者只是运行一个查询),将上面的数据转换成如下所示：

+----------+---------+-----------+-------+-----+
| 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   |
+----------+---------+-----------+-------+-----+

即,它是订单中每个商品的数量与产品名称的总和;并将产品名称设置为列标题.我是否需要使用交叉表这样的东西,或者使用子查询是否有一种聪明的方法,即使在查询运行之前我不知道不同产品名称的列表.

解决方法

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,'T-Shirt');
insert into orders_items VALUES (2,3,'Jeans');
insert into orders_items VALUES (3,'Hat');
insert into orders_items VALUES (4,2,'T-Shirt');
insert into orders_items VALUES (5,'T-Shirt');
insert into orders_items VALUES (6,'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,您可以尝试像这样使用tablefunc：

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
);

但我认为你正在考虑关于SQL的错误方法.您通常知道您想要哪些行,并且必须告诉它SQL. “旋转表”90度不是SQL的一部分,应该避免.

（编辑：李大同）

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