如何在postgreSQL中获取最后24小时的数据

Question

I want to get last 24 hrs data. i wrote a query in postgreSQL as follows. But I couldn't get the answer as i expected.

 SELECT startdate::timestamp AS startdate,
    (DATE_PART('hour',startdate::timestamp)::integer) as hrs,count(guorderid)
    FROM ord_entitlement
    WHERE DATE_PART('Day',CURRENT_DATE::timestamp - startdate::timestamp) < 1
    AND DATE_PART('hour',startdate::timestamp) <= 24
    GROUP BY  hrs,startdate
    ORDER BY startdate

Answer 1

Instead of checking date parts, do the time math to get an interval. Use NOW() to get a timestamptz.

SELECT startdate::timestamp AS startdate,
      (DATE_PART('hour',startdate::timestamp)::integer) as hrs, 
      count(guorderid)
FROM ord_entitlement
WHERE NOW() > startdate::timestamptz
  AND NOW() - startdate::timestamptz <= interval '24 hours'
GROUP BY hrs,startdate
ORDER BY startdate

This ensures you will get the last 24 hours no matter what your time zone or daylight savings says. NOW() > startdate::timestamptz ensures you don't accidentally pick up things from the future.

Answer 2

If you use CURRENT_DATE you will not get the time instead use now() function. Try the following,

        SELECT startdate::timestamp AS startdate,
        (DATE_PART('hour',startdate::timestamp)::integer) as hrs,count(guorderid)
        FROM ord_entitlement
        WHERE DATE_PART('Day',now() - startdate::timestamptz) < 1
        GROUP BY  hrs,startdate
        ORDER BY startdate

Answer 3

date_part() works like extract() , ie they will extract a subfield from the source:

-- they will both yield 9 as result
select date_part('day', date '2015-01-09') "day part of 2015-01-09",
       date_part('day', date '2015-02-09') "day part of 2015-02-09";

Extracting day(s) therefore is not suited to select the last 24 hours. Similarly extracting hour(s) will (almost) always yield less than or equal to 24 .

Extraction day(s) from interval (that's the result of substracting 2 timestamp s) is a little different. The result might depend on, whether the interval is justified, or not:

-- they will both yield 1 as result
select date_part('day', interval '1 day') "day part of 1 day",
       date_part('day', interval '1 month 1 day') "day part of 1 month 1 day";


-- they will yield 1, 32 and 397 respectively
select date_part('day', timestamp '2015-02-09' - timestamp '2015-02-08') "interval 1",
       date_part('day', timestamp '2015-02-09' - timestamp '2015-01-08') "interval 2",
       date_part('day', timestamp '2015-02-09' - timestamp '2014-01-08') "interval 3";

Depending on the fact, that the timestamp subtraction is not giving justified intervals is not the best option, I think. You could use simpler conditions to achieve your goal:

-- if startdate is a timestamp:
where current_timestamp - interval '1 day' <= startdate

-- if startdate is a date:
where current_date - 1 <= startdate

If you want to disallow future dates too (as your question's title suggests), you could use a single between condition:

-- if startdate is a timestamp:
where startdate between current_timestamp - interval '1 day' and current_timestamp

-- if startdate is a date:
where startdate between current_date - 1 and current_date