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Kôd:
# select * from test;
group_id | flag
----------+------
1 | no
1 | yes
2 | no
2 | no
1 | no
(5 rows)
# select t1.group_id, coalesce(t2.flag,'no') as f
from test t1
left join test t2 on t1.group_id = t2.group_id and t2.flag = 'yes'
group by t1.group_id, f;
group_id | f
----------+-----
1 | yes
2 | no
(2 rows)