PostgreSQL update field for each element in array - sql

Question

I wish to do the following task in SQL:
I have a table with columns:
uuid (uuid), word (text), wordList (text[]), uuidList (uuid[])
I have the wordList array, uuid and word columns populated. I wish to update and populate the uuidList like this:
foreach element in wordList
var x = select uuid where word = element;
uuidList.append(x);
Example:
I have a table like this:
+---------+-------+--------------------+----------+
| uuid | word | wordList | uuidList |
+---------+-------+--------------------+----------+
| aaaa... | hello | NULL | NULL |
| bbbb... | world | NULL | NULL |
| cccc... | blah | {'hello', 'world'} | NULL |
+---------+-------+--------------------+----------+
I want it to become like this:
+---------+-------+--------------------+--------------------+
| uuid | word | wordList | uuidList |
+---------+-------+--------------------+--------------------+
| aaaa... | hello | NULL | NULL |
| bbbb... | world | NULL | NULL |
| cccc... | blah | {'hello', 'world'} | {aaaa..., bbbb...} |
+---------+-------+--------------------+--------------------+
I'm quite new to SQL and have gotten confused how to do it. I don't think I can join a table to itself. I don't know if I should be storing information in a temporary table to somehow achieve this (some related questions I read had this proposed)...
Thanks!

Answer 1

You can aggregate all the needed UUIDs in a single statement:
select w1.uid, array_agg(w2.uid order by wl.idx) as uuidlist
from words w1
cross join lateral unnest(w1.wordlist) with ordinality as wl(word,idx)
join words w2 on w2.word = wl.word
where w1.wordlist is not null
and w1.uuidlist is null -- optional
group by w1.uid;
The option with ordinality returns an additional column that indicates the position of the element in the original array. This is needed to aggregate the UUIDs in the correct order.
This returns the following result with your sample data:
uid | uuidlist
-----+------------
cccc | {aaaa,bbbb}
This can be used as the source of an update statement (assuming the column uid is unique):
update words
set uuidlist = t.uuidlist
from (
select w1.uid, array_agg(w2.uid order by wl.idx) as uuidlist
from words w1
cross join lateral unnest(w1.wordlist) with ordinality as wl(word,idx)
join words w2 on w2.word = wl.word
where w1.wordlist is not null
and w1.uuidlist is null -- optional
group by w1.uid
) t
where t.uid = words.uid;
Online example: https://rextester.com/LZUYC57184
(note that the display of arrays is a bit weird in that example)