PostgreSQL: Updating column in a table based on another indirectly referenced table - postgresql

Question

After adding a new column new_column to an existing table rules, I would like to apply some complex migration logic in order to determine the new value for each row (I need to look at another table called flags which is indirectly referenced by rules).
My questions:
Q1. Can I accomplish this with JOIN and a CASE statement, or would I need to write a pgSQL function?
Q2. Which of the logical approaches is better (pseudo code):
FOR flag IN flags
FOR app IN applications WHERE app.accountid = flag.accountid
FOR campaign IN campaigns WHERE campaign.applicationid = app.id
FOR rule IN rules WHERE rule.campaignid = campaign.id
SET rule.new_column TO
(CASE flag.new == true AND flag.old == false THEN ‘v2’
CASE flag.new == false AND flag.old == true THEN ‘v1’
ELSE return ‘v0’)
Or is it better to go about it this way:
FOR rule IN rules
SELECT campaign FROM campaigns WHERE campaign.id = rule.campaignid
SELECT app FROM applications WHERE app.id = campaign.applicationid
SELECT flag FROM flags where flag.accountid = app.accountid
SET rule.new_column TO
CASE (flag.new = true AND flag.old = false) THEN 'v2'
CASE (flag.new = false AND flag.old = true) THEN 'v1'
ELSE 'v0'
Example:
flags:
accountid | new | old
---------------------
1 |true |true
2 |true |false
applications:
id | accountid
--------------
3 | 1
4 | 2
campaigns:
id | applicationid
------------------
5 | 3
6 | 4
rules:
campaignid | new_column (wanted result)
----------------------
5 | 'v0'
5 | 'v0'
6 | 'v2'

Answer 1

This should do it:
update rules
set new_column = case
when f.new and not f.old then 'v2'
when not f.new and f.old then 'v1'
else 'v0'
end
from campaigns c
join applications a on c.applicationid = a.id
join flags f on a.accountid = f.accountid
where c.id = rules.campaignid;
Online example: http://rextester.com/JHHP6967

Answer 2

I think I've achieved what I wanted with JOINed query expression (thanks #joop).
I'll post it here in case it might be useful for others:
UPDATE
rules AS r
SET
rbversion =
CASE
WHEN f.new = TRUE AND f.old = FALSE THEN 'v2'
WHEN f.new = FALSE AND f.old = TRUE THEN 'v1'
ELSE 'v0'
END
FROM
flags AS f,
campaigns AS c,
applications AS a
WHERE
c.id = r.campaignid AND
a.id = c.applicationid AND
f.accountid = a.accountid