Sample of an inline update from a reconciliation query on PostgreSQL 10

update-join-postgre.sql

create table bar (id int, top int, extid varchar)

insert into bar values
(1, 0, 'MC1234'),
(2, 1, 'FR1234'),
(3, 1, 'MC9876'),
(4, 0, 'MC5678')

commit

-- reconciliate rows MC1234 and FR1234
select * 
from bar a 
join bar b 
on substring(b.extid, 3, 4) = substring(a.extid, 3, 4) 
and substring(b.extid, 1, 2) <> substring(a.extid, 1, 2) 
and substring(a.extid, 1, 2) = 'FR'

-- inline update
update bar
set top = a.top | b.top
from bar a 
join bar b
on substring(b.extid, 3, 4) = substring(a.extid, 3, 4)
and substring(b.extid, 1, 2) <> substring(a.extid, 1, 2)
and substring(a.extid, 1, 2) = 'FR'
where bar.id = b.id