只有第一行的左外连接

Question

我有一个类似

的查询

SELECT S.product_id, S.link, C.id AS category_id
FROM Products P
INNER JOIN SEO S ON S.product_id = P.id AND P.product_type = 1
LEFT OUTER JOIN Categories C ON c.product_id = P.id
WHERE P.active = 1

只要每个产品只分配给一个类别，我就可以正常工作。但是，如果一个产品被分配到多个类别，它 returns 所有可能的组合。

我可以只 select 第一个吗，如果产品没有任何类别， link 仍然应该返回 category_id = NULL

Answer 1

您可以使用 GROUP BY to accomplish this along with an Aggregate function, most likely MIN or MAX.

根据您在结果中更喜欢的类别 ID，您可以 select 最小值。

SELECT S.product_id, S.link, MIN(C.id) AS category_id
FROM Products P
INNER JOIN SEO S ON S.product_id = P.id AND P.product_type = 1
LEFT OUTER JOIN Categories C ON c.product_id = P.id
WHERE P.active = 1
GROUP BY S.product_id, S.link

或最大值。

SELECT S.product_id, S.link, MAX(C.id) AS category_id
FROM Products P
INNER JOIN SEO S ON S.product_id = P.id AND P.product_type = 1
LEFT OUTER JOIN Categories C ON c.product_id = P.id
WHERE P.active = 1
GROUP BY S.product_id, S.link

Answer 2

使用子查询的替代解决方案：

SELECT S.product_id, S.link, 
(
 SELECT C.id FROM Categories C WHERE C.product_id = P.id AND
 ROW_NUMBER() OVER(ORDER BY /* your sort option goes here*/ ) = 1
) AS category_id
FROM Products P
INNER JOIN SEO S ON S.product_id = P.id AND P.product_type = 1
WHERE P.active = 1

Answer 3

一个简单的方法是使用外部应用，以便有一个相关联，并使它成为前 1 个查询。因此，您可以访问相关类别记录的所有列。我在这里添加一个类别名称作为示例：

select s.product_id, s.link, c.id as category_id, c.name as category_name
from products p
inner join seo s on s.product_id = p.id 
outer apply
(
  select top 1 * 
  from categories cat
  where cat.product_id = p.id
  order by cat.id
) c
where p.active = 1
and p.product_type = 1;

只有第一行的左外连接

Left outer join with only first row

sql

sql-server

greatest-n-per-group