I'm assuming you have rows in there "during the day" of Sept 17, maybe with times of 3 pm, ETC.

the thing to remember is that when you use '20130917' on a datetime column, there's an implied midnight. So your original query will only capture those rows that occurred on 9/17 at midnight...not ones that happened throughout the day.

You can try any of these...

select * from mytable where date_time >= '20130917' and date_time < '20130918'

or

select * from mytable where cast(date_time as date) = '20130917'

Though the first is likely to perform better than the 2nd....