Sql 服务器：限制 string_agg 结果

Question

我有以下查询（为每个客户显示用户列表）：

select cu.customer_id , STRING_AGG(u.first_name + ' ' + u.last_name , ',') as users   
from customer_user cu join user u on cu.user_id = u.id   
where ... 
group by cu.customer_id

如何限制 string_agg function 为每个组仅聚合 10 个元素？

Answer 1

您可以尝试对行进行编号：

SELECT customer_id , STRING_AGG(first_name + ' ' + last_name , ',') AS users   
FROM (
   SELECT 
      cu.customer_id, u.first_name, u.last_name,
      ROW_NUMBER() OVER (PARTITION BY cu.customer_id ORDER BY (SELECT NULL)) AS rn
   FROM customer_user cu 
   JOIN user u ON cu.user_id = u.id
   -- WHERE ...
) t
WHERE rn <= 10
GROUP BY customer_id

Answer 2

同样，这是另一种情况，我发现在 CTE 中分离逻辑的“丑陋”部分（串联并确定“第一个”或“任何”10），然后直到之后才聚合：

; -- see sqlblog.org/cte
WITH src AS
(
  SELECT cu.customer_id, n = CONCAT(u.first_name, ' ', u.last_name),
      rn = ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY @@SPID)
    FROM dbo.customer_user AS cu
    INNER JOIN dbo.[user] AS u -- bad table name
    ON cu.user_id = u.id
    /* WHERE ... */
)
SELECT customer_id, users = STRING_AGG(n, N',')
  FROM src
  WHERE rn <= 10
  GROUP BY customer_id;

• 示例数据库<>小提琴

Answer 3

我们可以在子查询中使用 row_number。 我在这个例子中限制为 2，您可以将 rn 的限制更改为 10 或其他数字。
我显示第一个查询没有限制，第二个查询有限制以显示差异。

 create table users( id int, first_name varchar(25), last_name varchar(25) ); insert into users values (1,'Andrew','A'), (2,'Bill','B'), (3,'Charlie','C'); create table customer_user( customer_id int, user_id int); insert into customer_user values (1,1),(1,2),(1,3); GO

6 行受影响

select cu.customer_id, STRING_AGG(u.first_name + ' ' + u.last_name, ',') as users from customer_user cu join users u on cu.user_id = u.id group by cu.customer_id

 GO

 客户编号 | 用户 ----------: |:------------------------ 1 | 安德鲁 A、比尔 B、查理 C

 select u.customer_id, STRING_AGG(u.first_name + ' ' + u.last_name, ',') as users from ( select row_number() over(partition by customer_id order by u.id) rn, cu.customer_id, u.first_name, u.last_name from customer_user cu join users u on cu.user_id = u.id ) u where rn < 3 group by u.customer_id GO

 客户编号 | 用户 ----------: |:------------ 1 | 安德鲁 A，比尔 B

db<> 在这里摆弄