为什么下面的查询不起作用

select count(*) from db.full where substr(starttime,1,10) < (select min(cast(substr(starttime,1,10) as date)) as min_date from db.v9_N where substr(starttime,1,10) between '2018-10-01' and '2018-10-10');

为什么下面的查询有效？

select count(*) from db.full where substr(starttime,1,10) in (select min(cast(substr(starttime,1,10) as date)) as min_date from db.v9_N where substr(starttime,1,10) between '2018-10-01' and '2018-10-10');

请帮助我理解？
如何使上述查询可行？