Postgres 中 12 月 31 日的年份数不一致... 1、52 ​​或 53

Question

在尝试按一年中的星期分组时，我发现 12 月 31 日在 Postgres 中返回的一年中的一周因年份而异，结果出乎意料（至少对我而言）。结果通常是 52（好），有时是 53，通常是 1。

53 我可以理解，因为它是根据一周中的特定日期计算周数，因此按该天计算可能会得到 53 周。不是很好，但可以理解。

但是为什么 Postgres 经常说 12 月 31 日是一年的第一周呢？这有什么意义？

例子：

      SELECT EXTRACT(WEEK FROM DATE '2000-12-31') AS "Week",'Week2000' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2001-12-31') AS "Week",'Week2001' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2002-12-31') AS "Week",'Week2002' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2003-12-31') AS "Week",'Week2003' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2004-12-31') AS "Week",'Week2004' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2005-12-31') AS "Week",'Week2005' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2006-12-31') AS "Week",'Week2006' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2007-12-31') AS "Week",'Week2007' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2008-12-31') AS "Week",'Week2008' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2009-12-31') AS "Week",'Week2009' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2010-12-31') AS "Week",'Week2010' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2011-12-31') AS "Week",'Week2011' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2012-12-31') AS "Week",'Week2012' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2013-12-31') AS "Week",'Week2013' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2014-12-31') AS "Week",'Week2014' AS "Year"
UNION SELECT EXTRACT(WEEK FROM DATE '2015-12-31') AS "Week",'Week2015' AS "Year"
ORDER BY "Year"

结果：

Week   Year
-----  ---------
52     Week2000
1      Week2001
1      Week2002
1      Week2003
53     Week2004
52     Week2005
52     Week2006
1      Week2007
1      Week2008
53     Week2009
52     Week2010
52     Week2011
1      Week2012
1      Week2013
1      Week2014
53     Week2015

我已经测试过指定完整的时间戳，而不仅仅是日期，并且无论我在时间戳中包含一天中的哪个小时，结果都相同。

12 月 29 日和 30 日也可能产生 1、52 ​​或 53 个结果。

12 月 27 日和 28 日可能导致 52 或 53。

Answer 1

PostgreSQL 使用 ISO8601 定义。这意味着周从星期一开始，一年中的第一周是 1 月 4 日所在的那一周。例如，如果 1 月 4 日是星期二，那么 12 月 31 日就是当年的第 1 周。