在 postgres 中获取月份的第一个日期 [英] Get first date of month in postgres

问题描述

我正在尝试获取对应于当月第一天的日期"类型.基本上我的一个表存储了一个日期，但我希望它始终是本月的第一个，所以我试图创建一个触发器，它将获取 now() 然后用 1 替换这一天.

解决方案

您可以使用表达式 date_trunc('month', current_date).用 SELECT 语句演示...

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

要删除时间，请投射到日期.

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

如果您确定该列应该总是存储一个月的第一天，您还应该使用 CHECK 约束.

创建表 foo (first_of_month 日期不为空检查(从 first_of_month 中提取(天)= 1));插入 foo (first_of_month) 值 ('2015-01-01');--成功插入 foo (first_of_month) 值 ('2015-01-02');--失败

<前>错误:关系foo"的新行违反检​​查约束foo_first_of_month_check"详细信息:失败的行包含 (2015-01-02).

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.

解决方案

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).

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