[英]SELECT TOP inside INNER JOIN
I created this simple database in SQL Server: 我在SQL Server中创建了这个简单的数据库:
create database product_test
go
use product_test
go
create table product
(
id int identity primary key,
label varchar(255),
description text,
price money,
);
create table picture
(
id int identity primary key,
p_path text,
product int foreign key references product(id)
);
insert into product
values ('flip phone 100', 'back 2 the future stuff.', 950),
('flip phone 200', 's;g material', 1400)
insert into picture
values ('1.jpg', 1), ('2.jpg', 1), ('3.jpg', 2)
What I want is to select all products and only one picture for each product. 我想要的是选择所有产品,每种产品只选择一张图片。 Any help is greatly appreciated.
任何帮助是极大的赞赏。
I'm a fan of outer apply
for this purpose: 我是
outer apply
的粉丝outer apply
这个目的:
select p.*, pi.id, pi.path
from product p outer apply
(select top 1 pi.*
from picture pi
where pi.product = p.id
) pi;
You can include an order by
to get one particular picture (say, the one with the lowest or highest id). 您可以通过包含
order by
来获取一张特定图片(例如,ID最低或最高的图片)。 Or, order by newid()
to get a random one. 或者,
order by newid()
命令获取随机的。
I would use a windowing function like this: 我会使用这样的窗口函数:
SELECT *
FROM product
JOIN (
SELECT id, product, p_path,
row_number() OVER (PARTITION BY product ORDER BY id ASC) as RN
FROM picture
) pic ON product.id = pic.product AND pic.RN = 1
As you can see here I am selecting the picture with the lowest id ( ORDER BY id ASC
) -- you can change this order by to your requirements. 正如您在这里看到的那样,我选择ID最低的图片(
ORDER BY id ASC
) - 您可以根据您的要求更改此订单。
SELECT
*,
(
SELECT TOP 1 p2.p_path
FROM dbo.picture p2
WHERE p.id = p2.product
) AS picture
FROM dbo.product p
Or with join: 或者加入:
SELECT
*
FROM dbo.product p
INNER JOIN
(
SELECT p2.product, MIN(p2.p_path) AS p_path
FROM dbo.picture p2
GROUP BY p2.product
) AS pt
ON p.id = pt.product
But you need to change p_path to varchar
type 但是您需要将p_path更改为
varchar
类型
Have you tried using a correlated sub-query? 您是否尝试过使用相关的子查询?
SELECT *, (SELECT TOP 1 p_path FROM picture WHERE product = p.id ORDER BY id)
FROM picture p
Hope this helps, 希望这可以帮助,
just group by and take min or max 只需分组并采取最小或最大
left join in case there is no picture 如果没有图片,请左连接
select pr.ID, pr.label, pr.text, pr.price
, min(pic.p_path)
from product pr
left join picture pic
on pic.product = pr.ID
group by pr.ID, pr.label, pr.text, pr.price
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