sql - 在 Informix 中将 DATETIME 转换为 Unix 纪元

Question

我有一个 DATETIME 列：

SELECT mytime FROM mytable;

mytime
--------------------
1/6/2013 10:41:41 PM

我想编写一个 SQL 语句，以 Unix 时间格式（自 Unix 纪元以来的秒数 - 01/01/1970 00:00:00）返回时间为INTEGER. 我曾尝试使用DATEDIFF，CAST但没有运气。这是Informix数据库。

Answer 1

假设该mytime列是 DATETIME YEAR TO SECOND 列（尽管问题中显示了格式），那么下面的存储过程就可以完成这项工作。它的评论比程序多，但评论解释了它在做什么。

{
#   "@(#)$Id: tounixtime.spl,v 1.6 2002/09/25 18:10:48 jleffler Exp $"
#
# Stored procedure TO_UNIX_TIME written by Jonathan Leffler (previously
# jleffler@informix.com and now jleffler@us.ibm.com).  Includes fix for
# bug reported by Tsutomu Ogiwara <Tsutomu.Ogiwara@ctc-g.co.jp> on
# 2001-07-13.  Previous version used DATETIME(0) SECOND TO SECOND
# instead of DATETIME(0:0:0) HOUR TO SECOND, and when the calculation
# extended the shorter constant to DATETIME HOUR TO SECOND, it added the
# current hour and minute fields, as documented in the Informix Guide to
# SQL: Syntax manual under EXTEND in the section on 'Expression'.
# Amended 2002-08-23 to handle 'eternity' and annotated more thoroughly.
# Amended 2002-09-25 to handle fractional seconds, as companion to the
# new stored procedure FROM_UNIX_TIME().
#
# If you run this procedure with no arguments (use the default), you
# need to worry about the time zone the database server is using because
# the value of CURRENT is determined by that, and you need to compensate
# for it if you are using a different time zone.
#
# Note that this version works for dates after 2001-09-09 when the
# interval between 1970-01-01 00:00:00+00:00 and current exceeds the
# range of INTERVAL SECOND(9) TO SECOND.  Returning DECIMAL(18,5) allows
# it to work for all valid datetime values including fractional seconds.
# In the UTC time zone, the 'Unix time' of 9999-12-31 23:59:59 is
# 253402300799 (12 digits); the equivalent for 0001-01-01 00:00:00 is
# -62135596800 (11 digits).  Both these values are unrepresentable in
# 32-bit integers, of course, so most Unix systems won't handle this
# range, and the so-called 'Proleptic Gregorian Calendar' used to
# calculate the dates ignores locale-dependent details such as the loss
# of days that occurred during the switch between the Julian and
# Gregorian calendar, but those are minutiae that most people can ignore
# most of the time.
}

CREATE PROCEDURE to_unix_time(d DATETIME YEAR TO FRACTION(5)
                                DEFAULT CURRENT YEAR TO FRACTION(5))
            RETURNING DECIMAL(18,5);
    DEFINE n DECIMAL(18,5);
    DEFINE i1 INTERVAL DAY(9) TO DAY;
    DEFINE i2 INTERVAL SECOND(6) TO FRACTION(5);
    DEFINE s1 CHAR(15);
    DEFINE s2 CHAR(15);
    LET i1 = EXTEND(d, YEAR TO DAY) - DATETIME(1970-01-01) YEAR TO DAY;
    LET s1 = i1;
    LET i2 = EXTEND(d, HOUR TO FRACTION(5)) -
                DATETIME(00:00:00.00000) HOUR TO FRACTION(5);
    LET s2 = i2;
    LET n = s1 * (24 * 60 * 60) + s2;
    RETURN n;
END PROCEDURE;

Answer 2

函数dbinfo('utc_current')返回纪元时间（自 1970-01-01 00:00:00 UTC 以来的秒数）。