STRING_AGG with CASE WHEN - sql-server

Question

The schema
CREATE TABLE person
(
[first_name] VARCHAR(10),
[surname] VARCHAR(10),
[dob] DATE,
[person_id] INT
);
INSERT INTO person ([first_name], [surname], [dob] ,[person_id])
VALUES
('Alice', 'AA', '1/1/1960', 1),
('Bob' , 'AA', '1/1/1980', 2),
('Carol', 'AA', '1/1/2018', 3),
('Dave' , 'BB', '1/1/1960', 4),
('Elsa', ' BB', '1/1/1980', 5),
('Fred' , 'BB', '1/1/1990', 6),
('Gina' , 'BB', '1/1/2018', 7);
CREATE TABLE person_membership
(
[person_id] INT,
[personstatus] VARCHAR(1),
[membership_id] INT,
[relationship] INT
);
INSERT INTO person_membership ([person_id], [personstatus], [membership_id], [relationship])
VALUES
(1, 'A', 10, 1),
(2, 'A', 10, 2),
(3, 'A', 10, 3),
(4, 'A', 20, 1),
(5, 'A', 20, 2),
(6, 'A', 20, 4),
(7, 'A', 20, 5);
In this simplified scheme the person with relationship set to 1 one is the main policy holder while different numbers show how other people are related to the main policy holder (spouse, children etc.)
The problem
Show all dependants for each main policy holder and group them within arbitrarily chosen age groups.
The desired output:
person_id|membership_id|first_name|dependants under 10|dependants over 10
---------+-------------+----------+-------------------+-------------------
1 | 10 | Alice | Bob | Carol
4 | 20 | Dave | Gina | Elsa, Fred
8 | 30 | Helen | Ida, Joe, Ken | NULL
My efforts so far:
SELECT
sub.person_id, sub.membership_id, sub.first_name,
STRING_AGG (sub.dependant, ',')
FROM
(SELECT
person.person_id, person_membership.membership_id,
person.first_name, p.first_name AS 'dependant',
DATEDIFF(yy, CONVERT(DATETIME, p.dob), GETDATE()) AS 'age'
FROM
person
LEFT JOIN
person_membership ON person.person_id = person_membership.person_id
LEFT JOIN
memship ON person_membership.membership_id = memship.membership_id
LEFT JOIN
person_membership pm ON person_membership.membership_id = pm.membership_id AND pm.relationship > 1
LEFT JOIN
person p ON pm.person_id = p.person_id
WHERE
person_membership.relationship = 1) as sub
GROUP BY
sub.person_id, sub.membership_id, sub.first_name
I can't figure out how to use CASE WHEN with STRING_AGG.
When I try something like
"CASE WHEN age < 10 THEN STRING_AGG (sub.dependant, ',') ELSE NULL END as 'Under 10'"
the server rightly protests that
contained in either an aggregate function or the GROUP BY clause
but of course grouping by it doesn't solve the problem either so there is a trick that I am missing. Also I'm sure it's possible to write the main query itself in a simpler way.
Edit - solution
As #Gserg rightly pointed out, and what I have realised moments after posting the question, the solution is very simple and calls for using CASE WHEN within STRING_AGG and not the other way around. Doh.
string_agg(case when age < 10 then sub.dependant else null end, ', ') as 'Under 10'
Still still looking for suggestions and ideas how to improve on my original query.

Answer 1

maximize using the iif function for a single condition.
SELECT sub.person_id, sub.membership_id, sub.first_name,
STRING_AGG (iif(age < 10, sub.dependant, null), ',') 'Under 10'
FROM (SELECT person.person_id, person_membership.membership_id, person.first_name, p.first_name AS 'dependant',
DATEDIFF(yy,CONVERT(DATETIME, p.dob),GETDATE()) AS 'age'
FROM person
LEFT JOIN person_membership ON person.person_id = person_membership.person_id
LEFT JOIN person_membership memship ON person_membership.membership_id = memship.membership_id
LEFT JOIN person_membership pm ON person_membership.membership_id = pm.membership_id AND pm.relationship > 1
LEFT JOIN person p ON pm.person_id = p.person_id
WHERE person_membership.relationship = 1) as sub
GROUP BY sub.person_id, sub.membership_id, sub.first_name