有表work_orders：
project_id step_id created_at
27983 523 2015-9-18 16:49
12981 133 2015-9-17 16:49
25145 523 2015-9-18 16:49
27983 523 2015-9-15 16:49
27983 624 2015-9-11 16:49
27983 535 2015-9-18 16:49
31266 528 2015-9-18 16:49
希望得到如下的结果：
project_id subcloning earliest_created_at
27983 YES 2015-9-11 16:49
12981
25145 2015-9-18 16:49
31266 YES
结果说明：若project_id对应的step_id是535或者528则subcloning项显示YES；step_id为523或者624的最早的那个created_at为earliest_created_at
------解决思路----------------------
select project_id
,subcloning=case when exists(select 1 from work_orders where project_id=t.project_id
and step_id in (535,528)) then 'YES' else '' end
,earliest_created_at=(select MIN(created_at) from work_orders where project_id=t.project_id
and step_id in (523,624))
from work_orders t
group by project_id