Postgres 12

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CREATE TABLE list_has_task (list_id int, task_id int, pos int);
INSERT INTO list_has_task VALUES
  (6969, 1001, 1)
, (6969, 1002, 2)
, (6969, 1003, 3)
, (6969, 1004, 4)
, (6969, 1005, 5)
;
5 rows affected

UPDATE list_has_task l
SET    pos = x.ord
FROM   unnest('{1004,1003,1002,1005,1001}'::int[]) WITH ORDINALITY x(task_id, ord)
WHERE  l.list_id = 6969
AND    l.task_id = x.task_id
;
5 rows affected

TABLE list_has_task ORDER BY pos;


list_id
task_id
pos


6969
1004
1

6969
1003
2

6969
1002
3

6969
1005
4

6969
1001
5