Postgres query with variable in loop and condition on variable - postgresql

Question

I have a query which updates the records based on variables old_id and new_id. But condition is I need to fetch the variables dynamically. Here is simple query which I am using.
do
$$
declare
old_id bigint = 1561049391647687270;
declare new_id bigint = 2068236279446765699;
begin
update songs set poet_id = new_id where poet_id = old_id;
update poets set active = true where id = new_id;
update poets set deleted = true where id = old_id;
end
$$;
I need to assign the old_id and new_id dynamically
do
$$
declare
su record;
pc record;
old_id bigint;
new_id bigint;
begin
for pc in select name, count(name)
from poets
where deleted = false
group by name
having count(name) > 1
order by name
loop
for su in select * from poets where name ilike pc.name
loop
-- old_id could be null where I have 2 continue the flow without update
for old_id in (select id from su where su.link is null)
loop
raise notice 'old: %', old_id;
end loop;
-- new_id could be more than 2 skip this condition as well
for new_id in (select id from su where su.link is not null)
loop
raise notice 'new: %', new_id;
end loop;
end loop;
-- run the statement_1 example if new_id and old_id is not null
end loop;
end
$$;
The expected problem statement (to assign variable and use it in further execution) is with in comment.

Answer 1

(a) In your first "simple query", the update of the table poets could be automatically executed by a trigger function defined on the table songs :
CREATE OR REPLACE FUNCTION songs_update_id ()
RETURNS trigger LANGUAGE plpgsql AS
$$
BEGIN
UPDATE poets SET active = true WHERE id = NEW.poet_id ;
UPDATE poets SET deleted = true WHERE id = OLD.poet_id ; -- SET active = false to be added ?
END ;
$$ ;
CREATE OR REPLACE TRIGGER songs_update_id AFTER UPDATE OF id ON songs
FOR EACH ROW EXECUTE songs_update_id () ;
Your first query can then be reduced as :
do
$$
declare
old_id bigint = 1561049391647687270;
declare new_id bigint = 2068236279446765699;
begin
update songs set poet_id = new_id where poet_id = old_id;
end
$$;
(b) The tables update could be performed with a sql query instead of a plpgsql loop and with better performances :
do
$$
BEGIN
UPDATE songs
SET poet_id = list.new_id[1]
FROM
( SELECT b.name
, array_agg(b.id) FILTER (WHERE b.link IS NULL) AS old_id
, array_agg(b.id) FILTER (WHERE b.link IS NOT NULL) AS new_id
FROM
( SELECT name
FROM poets
WHERE deleted = false
GROUP BY name
HAVING COUNT(*) > 1
-- ORDER BY name -- this ORDER BY sounds like useless and resource-intensive
) AS a
INNER JOIN poets AS b
ON b.name ilike a.name
GROUP BY b.name
HAVING array_length(old_id, 1) = 1
AND array_length(new_id, 1) = 1
) AS list
WHERE poet_id = list.old_id[1] ;
END ;
$$;
This solution is not tested yet and could have to be adjusted in order to work correctly. Please provide the tables definition of songs and poets and a sample of data in dbfiddle so that I can test and adjust the proposed solution.