Oracle条款在哪里的DateTime

Question

it seems there are lot of query syntax to fetch data on oracle database, here I just want to ask about the query that works fine but I cant understand at all. The query is :

Select
....
From
...
Where
TO_CHAR(TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS') +
(create_date  / ( 60 * 60 * 24 )),
'MM/DD/YY HH24:MI:SS') = '06/30/14 21:41:11'
;

From the query above it's work fine. But I cant understand why there's TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS and (create_date / ( 60 * 60 * 24 )), 'MM/DD/YY HH24:MI:SS')

on the create_date fields it show unix datetime such as 1404164471

Can anybody explain about this?

thanks in advance

Answer 1

TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS') converts a string (first argument) in certain format (second argument) to a date.

(create_date / ( 60 * 60 * 24 )) create_date contains seconds, this expression converts them into the number of days (1 minute = 60 seconds, 1 hour = 60 minutes, 1 day = 24 hours => 60*60*24 = the number of seconds in a day). When you add a number to a date Oracle thinks that this number contains days that's why you need such a conversation.

TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS') + (create_date / ( 60 * 60 * 24 )) gives you a date stored in create_date but in "traditional" format

It seems you need to compare unix time with date. It would be better to use this condition:

Select
....
From
...
Where create_date = trunc( (TO_DATE('06/30/14 21:41:11', 'MM/DD/YY HH24:MI:SS') 
                          - TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS')
                           ) * 24 * 60 * 60
                         );

Answer 2

The outer to_char(,) creates a string from the calculated date. This is so that it can be compared with the string '06/30/14 21:41:11'.

Inside the to_char some calculation is going on, the addition of to date values.

TO_DATE('01/01/1970 00:00:00', 'MM/DD/YYYY HH24:MI:SS') 
+ (create_date  / ( 60 * 60 * 24 ))

The to_date function takes a date value in string format ('01/01/1970 00:00:00'), and a format string ('MM/DD/YYYY HH24:MI:SS') to tell it how to interpret the date value.

create_date seems to be a number in seconds 9probably since 01-01-1970). It is devided by the number of seconds in a day so that will result in a number of days. So what you get is the create date in real calendar value.