SQL Date Format Conversion - sql

Question

I have a question regarding SQL dates.
The table I am working with has a date field in the following format: "22-SEP-08". The field is a date column.
I am trying to figure out how to output records from 1/1/2000 to present day.
The code below is not filtering the date field:
Select distinct entity.lt_date
from feed.entitytable entity
where entity.lt_date >= '2000-01-01'
Any help regarding this issue is much appreciated. Thanks!
Edit: I am using Oracle SQL Developer to write my code.

Answer 1

DATEs do not have "a format". Any format you see is applied by the application displaying the date value.
You can either change the configuration of SQL Developer to display dates in a different format, or you can use to_char() to format the date the way you want.
The reason your statement does not work, is most probably because of the implicit data type conversion that you are relying on.
'2000-01-01' is a string value, not a date. And the string is converted using the NLS settings of your session. Given the fact that you see dates displayed as DD-MON-YY means that that is the format that is used by the evil implicit data type conversion. You should supply date values always as real date literals.
There are two ways of specifying a real date literal. The first is ANSI SQL and simple uses the keyword DATE in front of an ISO formatted string:
where entity.lt_date >= DATE '2000-01-01'
Note the DATE keyword in front of the string, wich makes it a real date literal not a string expression.
The other option is to use to_date() to convert a character value into a date:
where entity.lt_date >= to_date('2000-01-01', 'yyyy-mm-dd');
More details about specifying date literals can be found in the manual:
Date literals
to_date function

Answer 2

My guess is the data type isn't a Date. Just in case its a char type, try to convert it using the Oracle TO_DATE() function. The Oracle documentation below should help you with parameters.
http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions183.htm

Answer 3

An implicit datatype conversion bites once again.
You're right. The predicate is not doing the comparison you are expecting,
Oracle is performing an implicit datatype conversion, from DATE to VARCHAR, so that it can do a comparison to the string literal.
If lt_date column is DATE datatype, then Oracle is seeing your where clause:
where entity.lt_date >= '2000-01-01'
Oracle is actually seeing it as if it's written like this:
where TO_CHAR(entity.lt_date) >= '2000-01-01'
And that's where the "format" problem comes in. The column itself does not have a "format". Because the second argument to the TO_CHAR function is not supplied, Oracle is using the value of the NLS_DATE_FORMAT parameter (from your session). And that's probably set to DD-MON-YY. Which is why that's the "format" you're seeing when you a run a SELECT statement in SQL*Plus. Because the DATE value is (again) being run through a TO_CHAR function to get a string that can be displayed.
To get the "filtering" you want, don't do a comparison to a string literal. Instead, do the comparison to an expression that has DATE datatype.
You can use the Oracle TO_DATE function. And you don't want to rely on setting of NLS_DATE_FORMAT, explicitly specify the format model as the second argument to the function. For example:
DO THIS
where entity.lt_date >= TO_DATE('2000-01-01','YYYY-MM-DD')
DON'T DO THIS
It's also possible to specify the format model as the second argument to the TO_CHAR function.
where TO_CHAR(entity.lt_date,'YYYY-MM-DD') >= '2001-01-01'
But you don't want to do that because that's going to force Oracle to evaluate that expression on the left side for every flipping row in the table, so it has a string value to do the comparison. (That's true unless someone created a function-based index for you.) If you do the comparison on the bare column, using the TO_DATE on the literal side, Oracle can make effective use of an appropriate index (with lt_date as the leading column) to satisfy the predicate.