Get first date of month in postgres

问题

I'm trying to get a 'date' type that corresponds to the first day of the current month. Basically one of my tables stores a date, but I want it to always be the first of the month, so I'm trying to create a trigger that will get now() and then replace the day with a 1.

回答1:

You can use the expression date_trunc('month', current_date). Demonstrated with a SELECT statement . . .

select date_trunc('month', current_date)
2013-08-01 00:00:00-04

To remove time, cast to date.

select cast(date_trunc('month', current_date) as date)
2013-08-01

If you're certain that column should always store only the first of a month, you should also use a CHECK constraint.

create table foo (
  first_of_month date not null
  check (extract (day from first_of_month) = 1)
);

insert into foo (first_of_month) values ('2015-01-01'); --Succeeds
insert into foo (first_of_month) values ('2015-01-02'); --Fails

ERROR:  new row for relation "foo" violates check constraint "foo_first_of_month_check"
DETAIL:  Failing row contains (2015-01-02).

回答2:

date_trunc() will do it.

SELECT date_trunc('MONTH',now())::DATE;

http://www.postgresql.org/docs/current/static/functions-datetime.html

回答3:

SELECT TO_DATE('2017-12-12', 'YYYY-MM-01');

2017-12-01

回答4:

You can also use TO_CHAR to get the first day of the month:

SELECT TO_CHAR(some_date, 'yyyy-mm-01')::date

回答5:

Found this to get the first day of that month and the last date of that month

select date_trunc('month', current_date-interval '1 year'), date_trunc('month', current_date-interval '1 year')+'1month'::interval-'1day'::interval;

来源：https://stackoverflow.com/questions/18069275/get-first-date-of-month-in-postgres