PostgreSQL update from subquery - postgresql

Question

I have a following PostgreSQL table:
CREATE TABLE users (
user_id INTEGER DEFAULT nextval('users_user_id_seq') NOT NULL,
user_old_id CHARACTER VARYING(36),
created_by INTEGER,
created_by_old character varying(36),
last_updated_by INTEGER,
last_updated_by_old character varying(36),
CONSTRAINT users_pkey PRIMARY KEY (user_id)
);
Based on data in this table I need to update:
created_by field with user_id from this table for every row
where created_by_old = user_old_id Please note that created_by_old can be NULL and therefore must be avoided in this case.
last_updated_by field with user_id from this table for every
row where last_updated_by_old = user_old_id Please note that last_updated_by_old can be NULL and therefore must be avoided in this case.
This is a sample data:
Actual:
user_id | user_old_id | created_by | created_by_old | last_updated_by | last_updated_by_old
--------------------------------------------------------------------------------------------
1 | aaa | | ccc | | bbb
--------------------------------------------------------------------------------------------
2 | bbb | | ddd | | aaa
--------------------------------------------------------------------------------------------
3 | ccc | | | | ddd
--------------------------------------------------------------------------------------------
4 | ddd | | aaa | |
Expected:
user_id | user_old_id | created_by | created_by_old | last_updated_by | last_updated_by_old
--------------------------------------------------------------------------------------------
1 | aaa | 3 | ccc | 2 | bbb
--------------------------------------------------------------------------------------------
2 | bbb | 4 | ddd | 1 | aaa
--------------------------------------------------------------------------------------------
3 | ccc | | | 4 | ddd
--------------------------------------------------------------------------------------------
4 | ddd | 1 | aaa | |
I think it can be implemented with a subquery but can't figure out by myself right now how to implement this query. Please help.

Answer 1

If you want a more efficient update statement you can use a derived table that gets the information you want as the source of your update statement:
update users u1
set created_by = t.new_created_by,
last_updated_by = t.new_updated_by
from (
select u2.user_id, u2.user_old_id,
cr.user_id as new_created_by,
lu.user_id as new_updated_by
from users u2
left join users cr on cr.user_old_id = u2.created_by_old
left join users lu on lu.user_old_id = u2.last_updated_by_old
) t
where t.user_id = u1.user_id;
Usually the target table of the update should never be repeated in the from clause, but this case is a rare example where it can't be avoided.
But this will only work properly if user_old_id is also unique, not only user_id.
Online example: http://rextester.com/HKM21985

Answer 2

Not too efficient ,but if that's not an issue you can do this :
UPDATE users u
SET u.created_by = (SELECT t.user_id FROM users t
WHERE u.created_by_old = t.user_old_id),
u.last_updated_by = (SELECT s.user_id FROM users s
WHERE u.last_updated_by = s.user_old_id)