Most efficient way of finding duplicates SQL Server - sql-server

Question

The fiddle:
CREATE TABLE person
([first_name] varchar(10), [surname] varchar(10), [date_of_birth] date, [person_id] int);
INSERT INTO person
([first_name], [surname], [date_of_birth] ,[person_id])
VALUES
('Alice', 'AA', '1/1/1990', 1),
('Bob' , 'BB', '1/1/1990', 3),
('Carol', 'CC', '1/1/1990', 4),
('Kate' , 'KK', '1/1/1990', 7);
CREATE TABLE person_membership
([person_id] int, [status_flag] varchar(1), [membership_id] int);
INSERT INTO person_membership
([person_id], [status_flag], [membership_id])
VALUES
(1, 'A', 10),
(1, 'A', 20),
(3, 'A', 30),
(4, 'A', 40),
(7, 'A', 60),
(7, 'T', 70);
CREATE TABLE memship
([membership_id] int, [memship_status] varchar(1));
INSERT INTO memship
([membership_id], [memship_status])
VALUES
(10, 'A'),
(20, 'A'),
(30, 'A'),
(40, 'A'),
(50, 'T'),
(60, 'A'),
(70, 'A');
The query:
WITH t AS
(SELECT first_name, surname, date_of_birth, p.person_id, m.membership_id
FROM person p
INNER JOIN person_membership pm ON p.person_id=pm.person_id
INNER JOIN memship m ON pm.membership_id = m.membership_id
WHERE pm.status_flag='A' and m.memship_status='A')
SELECT t.first_name, t.surname, t.date_of_birth, t.person_id, t1.membership_id
FROM t
INNER JOIN t t1 ON t.person_id=t1.person_id
GROUP BY t.first_name, t.surname, t.date_of_birth, t.person_id, t1.membership_id
HAVING count(*) > 1
The problem:
Find and display only those reconds marked as active and with multiple membership IDs assigned to one person id.
The expected outcome:
The question:
My query works fine and gives me the expected outcome but the execution plan looks rather convoluted. What are the better, more elegant, expert-recommended ways of doing it?

Answer 1

Seems like you don't need that big GROUP BY at all, you could use a windowed function inside the CTE instead:
WITH Counts AS(
SELECT p.first_name,
p.surname,
p.date_of_birth,
p.person_id,
m.membership_id,
COUNT(*) OVER (PARTITION BY p.person_id) AS PersonMemCount
FROM person p
INNER JOIN person_membership pm ON p.person_id=pm.person_id
INNER JOIN memship m ON pm.membership_id = m.membership_id
WHERE pm.status_flag='A'
AND m.memship_status='A')
SELECT C.first_name,
C.surname,
C.date_of_birth,
C.person_id,
C.membership_id
FROM Counts C
WHERE C.PersonMemCount > 1;