Postgres 14

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create table suspect_tracking (id int, city varchar(30))
insert into suspect_tracking values 
  (1, 'Brussels'), (1,'London'), (1,'Paris')
, (1,'New York'), (1,'Bangkok'), (1, 'Hong Kong')
, (1,'Dubai'), (1,'Singapoor'), (1,'Rome')
, (1,'Macau'), (1, 'Istanbul'), (1,'Kuala Lumpur')
, (1,'Dehli'), (1,'Tokyo'), (1,'Moscow')

, (2,'New York'), (2,'Bangkok'), (2, 'Hong Kong')
, (2,'Dubai'), (2,'Singapoor'), (2,'Rome')
, (2,'Macau'), (2, 'Istanbul'), (2,'Kuala Lumpur')

, (3,'Macau'), (3, 'Istanbul'), (3,'Kuala Lumpur')
, (3,'Dehli'), (3,'Tokyo'), (3,'Moscow')
30 rows affected

with cte_suspects as (
  select id, city
  from suspect_tracking
  group by id, city
)
, cte_prime_suspect as (
  select distinct id, city
  from suspect_tracking
  where id = 1
)
, cte_prime_total as (
  select id, count(city) as cities
  from cte_prime_suspect
  group by id
)
select sus.id
from cte_prime_suspect prime
join cte_prime_total primetot 
  on primetot.id = prime.id
join cte_suspects sus 
  on sus.city = prime.city and sus.id <> prime.id
group by prime.id, sus.id, primetot.cities
having count(sus.city) >= primetot.cities/2




id


2