Dates in concatinations

Question

I am running a query on a table with a primary key of column a, column b, and a date.
When I run a query concatinating all the columns of the key and counting the records with the same primary key combo.
For instance:
Select columnA||columnB||key_date, count(*)
from table
group by columnA||columnB||keydate
having count(*) > 1;
I get several rows with a count greater than 1.
When I look at the individual records you can see the time
stamp on the dates vary. But since I didn't do a trunc on the date; I didn't expect them to be grouped together. When you concatenate; do you lose the time stamp on the date?

Answer 1

If you rely on implicit conversions - which is what you are doing here - the date will get converted to a string using whatever default nls_date_format you have set, which is usually DD-MON-YY.
The key is to never rely on these implicit conversions - always convert dates to strings, etc. yourself.
sql>alter session set nls_date_format = 'DD-MON-YYYY';
Session altered.
sql>select 'a' || 'b' || sysdate from dual;
'A'||'B'||SYS
ab20-FEB-2003  -- see how the time gets dropped
1 row selected.
sql>select 'a' || 'b' || to_char(sysdate, 'yyyymmddhhmiss') from dual;
'A'||'B'||TO_CHA
ab20030220114814
1 row selected.