2 queries should return the same result (but they dont...)

Question

hello
i have a following query:
select col1,extract(year from datum) yr, COUNT(*)
from tableA@dblink where
DATUM between '1-jan-1985' and '31-dec-2012'
and col2 > 100 and col2 not in ('999999')
and TRIM(TO_CHAR(col1)) in ('0','1')
group by col1,extract(year from DATUM);the above query returns the count: 143 982 for year 1991
however when i put the filter directly into this year the query returns a different number: 143 917
select col1,extract(year from datum) yr, COUNT(*)
from tableA@dblink  where
DATUM between '1-jan-1991' and '31-dec-1991'
and col2 > 100 and col2 not in ('999999')
and TRIM(TO_CHAR(col1)) in ('0','1')
group by col1,extract(year from DATUM);please can you help me understand why is this happening and why these 2 counts are different?
id appreciate any tips
thanks very much
rgds

Answer 1

UserMB wrote:
please can you help me understand why is this happening and why these 2 counts are different?
id appreciate any tipsThis is tricky. All others of cause are right. You must compare date columns with date values.
If you don't explicitly compare as a date value, then it might happen that a string comparison is made.
And if that happens both of your counts would return a wrong result. For example all dates that start with
'4' are not counted. Like '4-jan-1991'. The string '4-jan-1991' is greater then the string '31-dec-1991'. Therefore it would not be included in the count.
I don't think this happened. But the danger is there.
However I have a problem seeing, what the difference between your two counts can be.
The two important parts are the date between filter and the group by condition.
DATUM between '1-jan-1985' and '31-dec-2012'
DATUM between '1-jan-1991' and '31-dec-1991'
group by col1,extract(year from DATUM);The second option returns less results than the first option. Some rows must be missing because of this different filter condition.
If this would be a text comparison then there shouldn't be a difference. Only if it is a date comparison then this could be explained.
Here is an example:
Dates like 31-dec-1991 17:53:14 are included in the first count+group, but not included in the second count.
Why? Because the string '31-dec-1991' is converted into a date. This date is 31-dec-1991 00:00:00 (midnight). Everything from this day, that is not exactly on midnight is greater than this value and therefore not included in your second query.
So as others already pointed out you need to either truncate the date or to compare it a little differently.
select col1,extract(year from datum) yr, COUNT(*)
from tableA@dblink 
where DATUM >= to_date('01-01-1991','dd-mm-yyyy')
and  DATUM < to_date('01-01-1992','dd-mm-yyyy')
and col2 > 100 and col2 not in ('999999')
and TRIM(TO_CHAR(col1)) in ('0','1')
group by col1,extract(year from DATUM);This should give the same result as your first query.
Note that I changed the month from a month name to a number. This makes it independent from national language settings. E.g. DEZ = German, DEC = American
Edited by: Sven W. on Oct 9, 2012 2:04 PM