Comparing Dates in Oracle SQL
I'm trying to get it to display the number of employees that are hired after June 20, 1994,
Select employee_id, count(*)
From Employee
Where to_char(employee_date_hired, 'DD-MON-YY') > 31-DEC-95;
But I get an error saying
"JUN' invalid identifier.
Please help, thanks!
31-DEC-95
isn't a string, nor is 20-JUN-94
. They're numbers with some extra stuff added on the end. This should be '31-DEC-95'
or '20-JUN-94'
- note the single quote, '
. This will enable you to do a string comparison.
However, you're not doing a string comparison; you're doing a date comparison. You should transform your string into a date. Either by using the built-in TO_DATE()
function, or a date literal.
TO_DATE()
select employee_id
from employee
where employee_date_hired > to_date('31-DEC-95','DD-MON-YY')
This method has a few unnecessary pitfalls
- As a_horse_with_no_name noted in the comments,
DEC
, doesn't necessarily mean December. It depends on yourNLS_DATE_LANGUAGE
andNLS_DATE_FORMAT
settings. To ensure that your comparison with work in any locale you can use the datetime format modelMM
instead - The year '95 is inexact. You know you mean 1995, but what if it was '50, is that 1950 or 2050? It's always best to be explicit
select employee_id
from employee
where employee_date_hired > to_date('31-12-1995','DD-MM-YYYY')
Date literals
A date literal is part of the ANSI standard, which means you don't have to use an Oracle specific function. When using a literal you must specify your date in the format YYYY-MM-DD
and you cannot include a time element.
select employee_id
from employee
where employee_date_hired > date '1995-12-31'
Remember that the Oracle date datatype includes a time elemement, so the date without a time portion is equivalent to 1995-12-31 00:00:00
.
If you want to include a time portion then you'd have to use a timestamp literal, which takes the format YYYY-MM-DD HH24:MI:SS[.FF0-9]
select employee_id
from employee
where employee_date_hired > timestamp '1995-12-31 12:31:02'
Further information
NLS_DATE_LANGUAGE
is derived from NLS_LANGUAGE
and NLS_DATE_FORMAT
is derived from NLS_TERRITORY
. These are set when you initially created the database but they can be altered by changing your inialization parameters file - only if really required - or at the session level by using the ALTER SESSION
syntax. For instance:
alter session set nls_date_format = 'DD.MM.YYYY HH24:MI:SS';
This means:
-
DD
numeric day of the month, 1 - 31 -
MM
numeric month of the year, 01 - 12 ( January is 01 ) -
YYYY
4 digit year - in my opinion this is always better than a 2 digit yearYY
as there is no confusion with what century you're referring to. -
HH24
hour of the day, 0 - 23 -
MI
minute of the hour, 0 - 59 -
SS
second of the minute, 0-59
You can find out your current language and date language settings by querying V$NLS_PARAMETERSs
and the full gamut of valid values by querying V$NLS_VALID_VALUES
.
Further reading
- Format models
Incidentally, if you want the count(*)
you need to group by employee_id
select employee_id, count(*)
from employee
where employee_date_hired > date '1995-12-31'
group by employee_id
This gives you the count per employee_id
.
Conclusion,
to_char
works in its own way
So,
Always use this format YYYY-MM-DD for comparison instead of MM-DD-YY or DD-MM-YYYY or any other format