time difference calculating postgres

• Displays the current time

  select now()
                now
  -------------------------------
   2019-10-19 01:03:13.525727+08
  (1 行记录)

• Calculate the time difference

  • The date_part () function. Separate part can return date

    select date_part('day',now())
    
     date_part
    -----------
            19
    (1 行记录)

    • Role: Calculation of poor, poor month, day difference, the difference between hours, minutes and poor

      select date_part('day',now()-create_time) from t_test (实际应用,看具体输出)
      
      select date_part('day',now()::timestamp-'2018-01-10 10:12:15::timestamp')
       date_part
      -----------
             646
      (1 行记录)

  • extract () function to calculate time difference

    select extract(day from now()-create_time) from t_test;

  • Add or subtract

    select now();
                  now
    -------------------------------
     2019-10-19 01:11:24.264592+08
    (1 行记录)
    select now()-interval '2 day';
               ?column?
    -------------------------------
     2019-10-17 01:11:24.264592+08
    (1 行记录)
    select now()+interval '2 month';
               ?column?
    -------------------------------
     2019-12-19 01:11:24.264592+08
    (1 行记录)

  • Calculating a first day of the month / week

  select date_trunc('month',now());
         date_trunc
  ------------------------
   2019-10-01 00:00:00+08
  (1 行记录)
  
  select date_trunc('week',now());
         date_trunc
  ------------------------
   2019-10-14 00:00:00+08
  (1 行记录)

• time unit

  • Year: year
  • Month: month (0-11)
  • S: second
  • Min: minutes (0-59)

• Reference documents:

  time difference calculating postgresql