I want to know the difference between Days column and Day1 Column

Question

Hi All,
I used this query:
SELECT to_char(return_date_time,'dd/mon/yyyy hh24:mi:ss') Test,
DECODE ('I', 'I', DECODE (1, NULL, NULL, 0, NULL, SYSDATE + 1)) Days,
DECODE ('I', 'I', SYSDATE + 1) Day1
FROM fm_curr_locn
WHERE patient_id = 'DU00002765'
output: 
Test
Days
Day1
26/oct/2013 00:00:00
26-OCT-13
10/26/2013 3:06:59 PM
I want to know the difference between Days column and Day1 column and why the days column didnt show the time
Please anyone help.....
Regards
Shagar M

Answer 1

Pleiadian wrote:
It is the decode statement that is doing this.
The format of decode is:
decode(expression, search1, result1, search2, result2, searchn, resultn, default)
All result fields must be of the same datatype and will be of the datatype of the first result (result1 in this example)
In your example (the decode for the field Days) the first result is a NULL. I suspect (gurus? anyone?) that Oracle will use the varchar2 overload of the decode statement, and the date field will be converted to varchar2 using your NLS_DATE_FORMAT settings.
In the second decode statment (Day1) the first result is of datatype date, so the result of the decode statement will be a date field (and will be parsed as such by your sql development tool)
I agree, as NULL is the first result returned the datatype is undetermined, so Oracle is picking VARCHAR2 over others, and causing an implicit datatype conversion on the resultant date value, whereas the other which is returned as a date will use the local settings of the client tool being used.
I can replicate the 'issue' in Toad, and it's rectified if we cast the first returned value of the decode statement...
SELECT DECODE ('I', 'I', DECODE (1, NULL, CAST(NULL AS DATE), 0, NULL, SYSDATE + 1)) Days,
       DECODE ('I', 'I', SYSDATE + 1) Day1
FROM dual