PostgreSQL group by column with aggregate

One method uses arrays:

SELECT id, 
       (ARRAY_AGG(task ORDER BY seq ASC))[1] as start_task,
       (ARRAY_AGG(task ORDER BY seq DESC))[1] as end_task
FROM table
GROUP BY id;

Another method uses window functions with SELECT DISTINCT:

select distinct id,
       first_value(task) over (partition by id order by seq) as start_task,
       first_value(task) over (partition by id order by seq desc) as end_task
from t;