Timestamp Difference In Hours for PostgreSQL

问题:

Is there a TIMESTAMPDIFF() equivalent for PostgreSQL?

I know I can subtract two timestamps to get a postgresql INTERVAL. I just want the difference between the two timestamps in in hours represented by an INT.

I can do this in MySQL like this:

TIMESTAMPDIFF(HOUR, links.created, NOW())

I just need the difference between two timestamps in hours represented as an integer.

Solution works for me:

SELECT "links_link"."created",
"links_link"."title",
(EXTRACT(EPOCH FROM current_timestamp - "links_link"."created")/3600)::Integer AS "age" 
FROM "links_link"

回答1:

The first things popping up

EXTRACT(EPOCH FROM current_timestamp-somedate)/3600

May not be pretty, but unblocks the road. Could be prettier if division of interval by interval was defined.

Edit: if you want it greater than zero either use abs or greatest(...,0). Whichever suits your intention.

Edit++: the reason why I didn't use age is that age with a single argument, to quote the documentation: Subtract from current_date (at midnight). Meaning you don't get an accurate "age" unless running at midnight. Right now it's almost 1am here:

select age(current_timestamp);
       age        
------------------
 -00:52:40.826309
(1 row)

回答2:

Get fields where a timestamp is greater than date in postgresql:

SELECT * from yourtable 
WHERE your_timestamp_field > to_date('05 Dec 2000', 'DD Mon YYYY');

Subtract minutes from timestamp in postgresql:

SELECT * from yourtable 
WHERE your_timestamp_field > current_timestamp - interval '5 minutes'

Subtract hours from timestamp in postgresql:

SELECT * from yourtable 
WHERE your_timestamp_field > current_timestamp - interval '5 hours'

回答3:

Michael Krelin's answer is close is not entirely safe, since it can be wrong in rare situations. The problem is that intervals in PostgreSQL do not have context with regards to things like daylight savings. Intervals store things internally as months, days, and seconds. Months aren't an issue in this case since subtracting two timestamps just use days and seconds but 'days' can be a problem.

If your subtraction involves daylight savings change-overs, a particular day might be considered 23 or 25 hours respectively. The interval will take that into account, which is useful for knowing the amount of days that passed in the symbolic sense but it would give an incorrect number of the actual hours that passed. Epoch on the interval will just multiply all days by 24 hours.

For example, if a full 'short' day passes and an additional hour of the next day, the interval will be recorded as one day and one hour. Which converted to epoch/3600 is 25 hours. But in reality 23 hours + 1 hour should be a total of 24 hours.

So the safer method is:

(EXTRACT(EPOCH FROM current_timestamp) - EXTRACT(EPOCH FROM somedate))/3600

As Michael mentioned in his follow-up comment, you'll also probably want to use floor() or round() to get the result as an integer value.

回答4:

You can use the "extract" or "date_part" functions on intervals as well as timestamps, but I don't think that does what you want. For example, it gives 3 for an interval of '2 days, 3 hours'. However, you can convert an interval to a number of seconds by specifying 'epoch' as the time element you want: extract(epoch from '2 days, 3 hours'::interval) returns 183600 (which you then divide by 3600 to convert seconds to hours).

So, putting this all together, you get basically Michael's answer: extract(epoch from timestamp1 - timestamp2)/3600. Since you don't seem to care about which timestamp precedes which, you probably want to wrap that in abs:

SELECT abs(extract(epoch from timestamp1 - timestamp2)/3600)

回答5:

postgresql get seconds difference between timestamps

SELECT (
    (extract (epoch from (
        '2012-01-01 18:25:00'::timestamp - '2012-01-01 18:25:02'::timestamp
                         )
             )
    )
)::integer

which prints:

-2

Because the timestamps are two seconds apart. Take the number and divide by 60 to get minutes, divide by 60 again to get hours.

回答6:

This might sound crazy to a lot of developers who like to take advantage of database functions,

But after exhaustive problems thinking, creating and bugfixing applications for mysql and postgrsql with php comparing date functions, I've come to the conclusion (for myself), that the easiest way, that is the simplest with less SQL headaches is not to take advantage of any of them.

Why? because if you are developing in a middleware language like PHP, PHP has all of these functions, and they are easier to implement in the application ode as comparing integers. PostgreSQL timestamp is NOT == UNIX TIMESTAMP and MySQL's UNIX TIMESTAMP is NOT PostgresQL's or Oracles timestamp.. it gets harder to port if you use database timestamps..

so just use an integer, not a timestamp, as the number of seconds since january 1st 1970 midnight. and never mind database timestamps. , and use gmdate() and store everything as gmt time to avoid timezone issues.

if you need to search, sort or compare the day from other data, or the month or the year or the day of the week, or anything, in your application, and INTEGER datatype for time_day, time_hour, time_seconds.. or whatever you wnat to index to be searched will make for smoother and more portable databases. you can just use one field, in most instances: INTEGER time_created NOT NULL

(more fields in your database row is the only drawback to this solution that i have found, and that doesnt cause as many headaches, or cups of coffee :)

php's date functions are outstanding to compare dates, but in mysql or postgresql, comparing dates ? nah.. use integer sql comparisons

i realize it may SEEM easier to use CURRENT_TIMESTAMP on an insert function. HA! don't be fooled.

You cant do DELETE FROM SESSION_TABLE WHERE time-initialized < '2 days' if time-intitialized is a postgresql timestamp. but you CAN do:

DELETE FROM SESSION_TABLE WHERE time_initialized < '$yesterday'

As long as you set $yesterday in php as the integer of seconds since 1970 that yesterday was.

This is easier housekeeping of session records than comparing timestamps in postgresql select statements.

SELECT age(), SELECT extract(), and asbtime are headaches in an of themselves. this is just my opinion.

you can do addition, substraction, <, >, all with php date objects

_peter_sysko U4EA Networks, Inc.

回答7:

extract(hour from age(now(),links.created)) gives you a floor-rounded count of the hour difference.