SQL Select Query on Date/Time column strange behavior - sql

Question

I am having an odd issue with a select statement in SQL Server Express, hopefully someone can shed some light on why I'm experiencing this behavior as well as the right way to do it.
When I run this query it returns exactly what I would expect:
SELECT
Action, TimeOccurred, UserName, IPv4From, ShareName,
FullFilePath, NewPathName, FromServer
FROM
File_System_Profiles
WHERE
Action LIKE '%create%'
AND (TimeOccurred >= '04/27/2017')
All those entries from 4/27 and anything after that.
When I run this query it returns 0 results which is extremely odd since there are entries for 4/27 in the previous query's results:
SELECT
Action, TimeOccurred, UserName, IPv4From, ShareName,
FullFilePath, NewPathName, FromServer
FROM
File_System_Profiles
WHERE
Action LIKE '%create%'
AND (TimeOccurred = '04/27/2017')
All I removed was the > between the = and the date and I get no results. I can clearly see when I run the first query that there are results where 04/27/2017 is the date that something occurred. If the first query didn't work I would assume there was a problem with my date format of MM/DD/YYYY and what is actually in the column YYYY-MM-DD HH:MM:SS but since the first one works it seems logical that the second one should.

Answer 1

It is a problem of the date format. As you mentioned, you are using Date format as "YYYY-MM-DD HH:MM:SS" so you should know that the value "04/27/2017" will be taken as "04/27/2017 00:00:00" when used in the comparison.
I assume that you have no record where the value in date field is "04/27/2017 00:00:00" and that is why when you use = operator, it does not match any record. However, the operator >= picks the records where hour:minute:second part of the date exist. As an example "04/27/2017 01:10:00" is greater than "04/27/2017 00:00:00" and therefore the > operator fetches those records.
Hope it helps.

Answer 2

First, always use ISO standard formats for comparison. Your code should look more like this:
where Action LIKE '%create%' AND
TimeOccurred = '2017-04-27'
Nauman explained the problem, which is the time component on TimeOccurred. You can solve this in various ways. One method is to convert to date. Something like this:
where Action LIKE '%create%' AND
cast(TimeOccurred as date) = '2017-04-27'
Depending on the database, the cast() might be replaced by trunc() or date_trunc() or something else.
However, I prefer this version:
where Action LIKE '%create%' AND
TimeOccurred >= '2017-04-27' AND
TimeOccurred < '2017-04-28'
It gets to the heart of the problem. And -- better yet -- SQL optimizers can make use of an index.