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yeah. this is a known issue - https://github.com/apache/druid/issues/13626

@Rahul Sharma, if you have to get this to work ...

with tmp as (select CURRENT_TIMESTAMP val)
select sum(added) ,
case when flags = 'MB' then tmp.val else null end grp
from "wikipedia" CROSS JOIN tmp
group by 2

it's a bit of a hack, but it is valid SQL and it works for me.