sql - Convert DATETIME to Unix Epoch in Informix

Question

I have a DATETIME column:

SELECT mytime FROM mytable;

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

I would like to write a SQL statement that returns the time in Unix Time format (seconds since Unix Epoch - 01/01/1970 00:00:00) as INTEGER. I have tried to use DATEDIFF and CAST but no luck. This is Informix database.

Answer 1

Assuming that the mytime column is a DATETIME YEAR TO SECOND column (despite the formatting shown in the question), then the following stored procedure does the job. It has more comment than procedure, but the comments explain what it is doing.

{
#   "@(#)$Id: tounixtime.spl,v 1.6 2002/09/25 18:10:48 jleffler Exp $"
#
# Stored procedure TO_UNIX_TIME written by Jonathan Leffler (previously
# [email protected] and now [email protected]).  Includes fix for
# bug reported by Tsutomu Ogiwara <[email protected]> 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;