Question

我在Postgres中有一个名为calendar的表，我正在尝试查询存储在year = 2013年的第一个日期值。

我已经编写了以下查询，但是我想知道我是否可以查询第一个日期而不是仅获取所有数据并仅限制为一个？

SELECT date FROM calendar WHERE year=2013 ORDER BY date ASC LIMIT 1

Answer 1

        -- the schema
DROP SCHEMA lutser CASCADE;
CREATE SCHEMA lutser;
SET search_path='lutser';

        -- the table
CREATE TABLE years
  ( zdate DATE NOT NULL PRIMARY KEY
  -- omitting a separate "year" field
  -- , since it can be derived from a date.
  -- ... ...
  );
        -- the data
INSERT INTO years(zdate)
SELECT gs
FROM generate_series( '2012-1-1' , '2014-12-31', '7 days'::interval) gs
        ;

        -- the query
SELECT zdate
FROM years yy
WHERE date_trunc( 'year' , yy.zdate) = '2014-01-01'::date
AND NOT EXISTS (
        SELECT *
        FROM years nx
        WHERE  date_trunc( 'year' , nx.zdate) = '2014-01-01'::date
        AND nx.zdate < yy.zdate
        )
        ;

        -- the same query using a CTE
WITH this AS (
        SELECT zdate
        FROM years yy
        WHERE date_trunc( 'year' , yy.zdate) = '2014-01-01'::date
        )
SELECT th.zdate
FROM this th
WHERE NOT EXISTS (
        SELECT *
        FROM this nx
        WHERE nx.zdate < th.zdate
        )
        ;


\q
        -- the result
DROP SCHEMA
CREATE SCHEMA
SET
CREATE TABLE
INSERT 0 157
   zdate
------------
 2014-01-05
(1 row)

Answer 2

您是否尝试过这样的事情？

如果您有年份字段，则为

select MIN(date)
from calendar
where year = 2013

如果您没有年份字段，则为

select MIN(date)
from calendar
where date >= '01-01-2013'

PostgreSQL查询日期最早的

2 个答案: