oracle的date

Querying by Date

Suppose that we have the following table to record user registrations:

create table users (
	user_id			integer primary key,
	first_names		varchar(50),
	last_name		varchar(50) not null,
	email			varchar(100) not null unique,
	-- we encrypt passwords using operating system crypt function
	password		varchar(30) not null,
	-- we only need precision to within one second
	registration_date	timestamp(0)
);

-- add some sample data 
insert into users
(user_id, first_names, last_name, email, password, registration_date)
values
(1,'schlomo','mendelowitz','[email protected]','67xui2',
to_timestamp('2003-06-13 09:15:00','YYYY-MM-DD HH24:MI:SS'));

insert into users
(user_id, first_names, last_name, email, password, registration_date)
values
(2,'George Herbert Walker','Bush','[email protected]','kl88q',
to_timestamp('2003-06-13 15:18:22','YYYY-MM-DD HH24:MI:SS'));

Let's query for people who registered during the last day:

column email format a35
column registration_date format a25

select email, registration_date 
from users
where registration_date > current_date - interval '1' day;

EMAIL                               REGISTRATION_DATE
----------------------------------- -------------------------
[email protected]             13-JUN-03 09.15.00 AM
[email protected]     13-JUN-03 03.18.22 PM

Note how the registration date comes out in a non-standard format that won't sort lexicographically and that does not have a full four digits for the year. You should curse your database administrator at this point for not configuring Oracle with a more sensible default. You can fix the problem for yourself right now, however:

alter session
set nls_timestamp_format =
'YYYY-MM-DD HH24:MI:SS';

select email, registration_date 
from users
where registration_date > current_date - interval '1' day;

EMAIL                               REGISTRATION_DATE
----------------------------------- ----------------------
[email protected]             2003-06-13 09:15:00
[email protected]     2003-06-13 15:18:22

You can query for shorter time intervals:

select email, registration_date 
from users
where registration_date > current_date - interval '1' hour;

EMAIL                               REGISTRATION_DATE
----------------------------------- -------------------------
[email protected]     2003-06-13 15:18:22

select email, registration_date 
from users
where registration_date > current_date - interval '1' minute;

no rows selected

select email, registration_date 
from users
where registration_date > current_date - interval '1' second;

no rows selected

You can be explicit about how you'd like the timestamps formatted:

select email, to_char(registration_date,'Day, Month DD, YYYY') as reg_day
from users
order by registration_date;

EMAIL                               REG_DAY
----------------------------------- -----------------------------
[email protected]             Friday   , June      13, 2003
[email protected]     Friday   , June      13, 2003

Oops. Oracle pads some of these fields by default so that reports will be lined up and neat. We'll have to trim the strings ourselves:

select 
  email, 
  trim(to_char(registration_date,'Day')) || ', ' ||
   trim(to_char(registration_date,'Month')) || ' ' ||
   trim(to_char(registration_date,'DD, YYYY')) as reg_day
from users
order by registration_date;

EMAIL                               REG_DAY
----------------------------------- ----------------------------
[email protected]             Friday, June 13, 2003
[email protected]     Friday, June 13, 2003

Some Very Weird Things

One reason that Oracle may have resisted ANSI date-time datatypes and arithmetic is that they can make life very strange for the programmer.

alter session set nls_date_format = 'YYYY-MM-DD';

-- old 
select add_months(to_date('2003-07-31','YYYY-MM-DD'),-1) from dual;

ADD_MONTHS
----------
2003-06-30

-- new
select to_timestamp('2003-07-31','YYYY-MM-DD') - interval '1' month from dual;

ERROR at line 1:
ORA-01839: date not valid for month specified

-- old 
select to_date('2003-07-31','YYYY-MM-DD') - 100 from dual;

TO_DATE('2
----------
2003-04-22

-- new (broken)
select to_timestamp('2003-07-31','YYYY-MM-DD') - interval '100' day from dual;

ERROR at line 1:
ORA-01873: the leading precision of the interval is too small

-- new (note the extra "(3)")
select to_timestamp('2003-07-31','YYYY-MM-DD') - interval '100' day(3) from dual;

TO_TIMESTAMP('2003-07-31','YYYY-MM-DD')-INTERVAL'100'DAY(3)
-------------------------------------------------------------
2003-04-22 00:00:00

Some Profoundly Painful Things

Calculating time intervals between rows in a table can be very painful because there is no way in standard SQL to refer to "the value of this column from the previous row in the report". You can do this easily enough in an imperative computer language, e.g., C#, Java, or Visual Basic, that is reading rows from an SQL database but doing it purely in SQL is tough.

Let's add a few more rows to our users table to see how this works.

insert into users
(user_id, first_names, last_name, email, password, registration_date)
values
(3,'Osama','bin Laden','[email protected]','dieusa',
to_timestamp('2003-06-13 17:56:03','YYYY-MM-DD HH24:MI:SS'));

insert into users
(user_id, first_names, last_name, email, password, registration_date)
values
(4,'Saddam','Hussein','[email protected]','wmd34',
to_timestamp('2003-06-13 19:12:43','YYYY-MM-DD HH24:MI:SS'));

Suppose that we're interested in the average length of time between registrations. With so few rows we could just query all the data out and eyeball it:

select registration_date
from users
order by registration_date;

REGISTRATION_DATE
-------------------------
2003-06-13 09:15:00
2003-06-13 15:18:22
2003-06-13 17:56:03
2003-06-13 19:12:43

If we have a lot of data, however, we'll need to do a self-join.

column r1 format a21
column r2 format a21

select 
  u1.registration_date as r1,
  u2.registration_date as r2
from users u1, users u2
where u2.user_id = (select min(user_id) from users 
                    where registration_date > u1.registration_date)
order by r1;

R1                    R2
--------------------- ---------------------
2003-06-13 09:15:00   2003-06-13 15:18:22
2003-06-13 15:18:22   2003-06-13 17:56:03
2003-06-13 17:56:03   2003-06-13 19:12:43

Notice that to find the "next row" for the pairing we are using the user_id column, which we know to be sequential and unique, rather than the registration_date column, which may not be unique because two users could register at exactly the same time.

Now that we have information from adjacent rows paired up in the same report we can begin to calculate intervals:

column reg_gap format a21

select 
  u1.registration_date as r1,
  u2.registration_date as r2,
  u2.registration_date-u1.registration_date as reg_gap
from users u1, users u2
where u2.user_id = (select min(user_id) from users 
                    where registration_date > u1.registration_date)
order by r1;

R1                    R2                    REG_GAP
--------------------- --------------------- ---------------------
2003-06-13 09:15:00   2003-06-13 15:18:22   +000000000 06:03:22
2003-06-13 15:18:22   2003-06-13 17:56:03   +000000000 02:37:41
2003-06-13 17:56:03   2003-06-13 19:12:43   +000000000 01:16:40

The interval for each row of the report has come back as days, hours, minutes, and seconds. At this point you'd expect to be able to average the intervals:

select avg(reg_gap)
from 
(select 
  u1.registration_date as r1,
  u2.registration_date as r2,
  u2.registration_date-u1.registration_date as reg_gap
from users u1, users u2
where u2.user_id = (select min(user_id) from users 
                    where registration_date > u1.registration_date))

ERROR at line 1:
ORA-00932: inconsistent datatypes: expected NUMBER got INTERVAL

Oops. Oracle isn't smart enough to aggregate time intervals. And sadly there doesn't seem to be an easy way to turn a time interval into a number of seconds, for example, that would be amenable to averaging. If you figure how out to do it, please let me know!

Should we give up? If you have a strong stomach you can convert the timestamps to old-style Oracle dates through character strings before creating the intervals. This will give us a result as a fraction of a day:

select avg(reg_gap)
from 
(select 
  u1.registration_date as r1,
  u2.registration_date as r2,
  to_date(to_char(u2.registration_date,'YYYY-MM-DD HH24:MI:SS'),'YYYY-MM-DD HH24:MI:SS')
   - to_date(to_char(u1.registration_date,'YYYY-MM-DD HH24:MI:SS'),'YYYY-MM-DD HH24:MI:SS')
   as reg_gap
from users u1, users u2
where u2.user_id = (select min(user_id) from users 
                    where registration_date > u1.registration_date))

AVG(REG_GAP)
------------
   .13836034

If we're going to continue using this ugly query we ought to create a view:

create view registration_intervals
as
select 
  u1.registration_date as r1,
  u2.registration_date as r2,
  to_date(to_char(u2.registration_date,'YYYY-MM-DD HH24:MI:SS'),'YYYY-MM-DD HH24:MI:SS')
   - to_date(to_char(u1.registration_date,'YYYY-MM-DD HH24:MI:SS'),'YYYY-MM-DD HH24:MI:SS')
   as reg_gap
from users u1, users u2
where u2.user_id = (select min(user_id) from users 
                    where registration_date > u1.registration_date)

Now we can calculate the average time interval in minutes:

select 24*60*avg(reg_gap) as avg_gap_minutes from registration_intervals;

AVG_GAP_MINUTES
---------------
     199.238889

Reporting

Here's an example of using the to_char function an GROUP BY to generate a report of sales by calendar quarter:

select to_char(shipped_date,'YYYY') as shipped_year, 
       to_char(shipped_date,'Q') as shipped_quarter, 
       sum(price_charged) as revenue
from sh_orders_reportable
where product_id = 143
and shipped_date is not null
group by to_char(shipped_date,'YYYY'), to_char(shipped_date,'Q')
order by to_char(shipped_date,'YYYY'), to_char(shipped_date,'Q');

SHIPPED_YEAR	     SHIPPED_QUARTER	     REVENUE
-------------------- -------------------- ----------
1998		     2				1280
1998		     3				1150
1998		     4				 350
1999		     1				 210

This is a hint that Oracle has all kinds of fancy date formats (covered in their online documentation). We're using the "Q" mask to get the calendar quarter. We can see that this product started shipping in Q2 1998 and that revenues trailed off in Q4 1998.