Sql server: limit string_agg result

Question

I have the following query (showing for each customer the list of users):

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

How can I limit the string_agg function to aggregate only 10 elements for each group?

Answer 1

You may try to number the rows:

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

Again this is another case where I find separating the "ugly" part of the logic (concatenation and determining "first" or "any" 10) in a CTE, then not aggregating until after:

; -- 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;

• Example db<>fiddle

Answer 3

We can use row_number in a sub-query. I have limited to 2 in this example and you can change the limit for rn as to 10, or other number.
I show the first query without a limit and the second with a limit to show the difference.

 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 rows affected

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

 customer_id | users ----------: |:------------------------ 1 | Andrew A,Bill B,Charlie 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

 customer_id | users ----------: |:-------------- 1 | Andrew A,Bill B

db<>fiddle here