DATE and DATETIME data types

Informix®

Informix provides two data types to store date and time information:

  • DATE = for year, month and day storage.
  • DATETIME = for year to fraction (1-5) storage.

The DATE type is stored as an INTEGER with the number of days since 1899/12/31.

The DATETIME type can be defined with various time units, by specifying a start and end qualifier. For example, you can define a datetime to store an hour-to-second time value with DATETIME HOUR TO SECOND.

The values of Informix DATETIME can be represented with a character string literal, or as DATETIME() literals:
'2017-12-24 15:45:12.345'  -- a DATETIME YEAR TO FRACTION(3)
'15:45'   -- a DATETIME HOUR TO MINUTE
DATETIME(2017-12-24 12:45) YEAR TO MINUTE
DATETIME(12:45:56.333) HOUR TO FRACTION(3)
Informix is able to convert quoted strings to DATE / DATETIME data, if the string contains matching environment parameters. The string to date conversion rules for DATE is defined by the DBDATE environment variable. The string to datetime format for DATETIME is defined by the GL_DATETIME environment variable.
Note: Within Genero programs, the string representation for DATETIME values is always ISO (YYYY-MM-DD hh:mm:ss.fffff)

Informix supports date arithmetic on DATE and DATETIME values. The result of an arithmetic expression involving dates/times is an INTEGER number of days when only DATE values are used, and an INTERVAL value if a DATETIME is used in the expression.

Informix automatically converts an INTEGER to a DATE when the integer is used to set a value of a date column.

Informix provides the CURRENT [ q1 TO q2 ] operator, to get the system date/time on the server where the current database is located. When no qualifiers are specified, CURRENT returns a DATETIME YEAR TO FRACTION(3). Informix also supports the SYSDATE operator, which returns the current system time as a DATETIME YEAR TO FRACTION(5).
Note: The USEOSTIME configuration parameter must be set to 1 in order to get the subsecond precision in CURRENT and SYSDATE operators. See Informix documentation for more details.

ORACLE

Oracle® provides the following data types to store date and time data:

  • DATE = for year, month, day, hour, min, second storage.
  • TIMESTAMP= for year, month, day, hour, min, second, fraction storage.

As in Informix, Oracle can convert quoted strings to DATE or TIMESTAMP data if the contents of the string matches the NLS date format parameters (NLS_DATE_FORMAT, NLS_TIMESTAMP_FORMAT).

The TO_DATE() and TO_TIMESTAMP() SQL functions convert strings to dates or timestamps, based on a given format. The TO_CHAR() SQL function allows you to convert dates or timestamps to strings, according to a given format.

In Oracle the result of an arithmetic expression involving DATE values is a number of days as NUMBER type; the decimal part is the fraction of the day ( 0.5 = 12H00, 2.00694444 = (2 + (10/1440)) = 2 days and 10 minutes ). The result of an expression involving Oracle TIMESTAMP data is of type INTERVAL.

To compare dates that have time data in Oracle, you can use the ROUND() or TRUNC() SQL functions.

Even if the keyword is the same, the Oracle SYSDATE operator (using second precision) is not the exact equivalent of the Informix SYSDATE operator (using subsecond precision). Use Oracle's SYSTIMESTAMP as equivalent for Informix SYSDATE operator.

See the Oracle documentation for more details.

Solution

Use the following conversion rules to map Informix date/time types to Oracle date/time types:

Table 1. Informix data types and Oracle equivalents
Informix data type Oracle
DATE DATE
DATETIME YEAR TO YEAR DATE
DATETIME YEAR TO MONTH DATE
DATETIME YEAR TO DAY DATE
DATETIME YEAR TO HOUR DATE
DATETIME YEAR TO MINUTE DATE
DATETIME YEAR TO SECOND DATE
DATETIME YEAR TO FRACTION(n) TIMESTAMP(n)
DATETIME MONTH TO MONTH DATE
DATETIME MONTH TO DAY DATE
DATETIME MONTH TO HOUR DATE
DATETIME MONTH TO MINUTE DATE
DATETIME MONTH TO SECOND DATE
DATETIME MONTH TO FRACTION(n) TIMESTAMP(n)
DATETIME DAY TO DAY DATE
DATETIME DAY TO HOUR DATE
DATETIME DAY TO MINUTE DATE
DATETIME DAY TO SECOND DATE
DATETIME DAY TO FRACTION(n) TIMESTAMP(n)
DATETIME HOUR TO HOUR DATE
DATETIME HOUR TO MINUTE DATE
DATETIME HOUR TO SECOND DATE
DATETIME HOUR TO FRACTION(n) TIMESTAMP(n)
DATETIME MINUTE TO MINUTE DATE
DATETIME MINUTE TO SECOND DATE
DATETIME MINUTE TO FRACTION(n) TIMESTAMP(n)
DATETIME SECOND TO SECOND DATE
DATETIME SECOND TO FRACTION(n) TIMESTAMP(n)
DATETIME FRACTION TO FRACTION(n) TIMESTAMP(n)
The DATE and DATETIME types translation can be controlled with the following FGLPROFILE entries:
dbi.database.dsname.ifxemul.datatype.date = { true | false }
dbi.database.dsname.ifxemul.datatype.datetime = { true | false }
For more details see IBM Informix emulation parameters in FGLPROFILE.

Storing BDL DATE values

The Oracle DATE type is used to store Genero BDL DATE values. However, keep in mind that the Oracle DATE type stores also time (hh:mm:ss) information. The database interface automatically sets the time part to midnight (00:00:00) during input/output operations.

You must be very careful since manual modifications of the database might set the time part, for example:
UPDATE table SET date_col = SYSDATE
(SYSDATE is equivalent to CURRENT YEAR TO SECOND in Informix).

After this type of update, when columns have date values with a time part different from midnight, some SELECT statements might not return all the expected rows.

When fetching Oracle DATE values into Genero BDL DATE or DATETIME variables, the date and time information is directly set for the individual date/time parts and the conversion is straight forward. But when fetching an Oracle DATE into a CHAR or VARCHAR variable, date to string conversion occurs. Since Oracle dates are equivalent of Informix DATETIME YEAR TO SECOND, the values are by default converted with the ISO format (YYYY-MM-DD hh:mm:ss), which is not the typical Informix behavior where dates are formatted from the DBDATE environment variable. If your application fetches DATE values into CHAR/VARCHAR and you want to get the DBDATE conversion, you must set the following FGLPROFILE entry:
dbi.database.dbname.ora.date.ifxfetch = true
Note: Oracle does not support INTEGER to DATE automatic conversion.

Storing BDL DATETIME values

Informix DATETIME data with any precision from YEAR to SECOND is stored in Oracle DATE columns. The database interface makes the conversion automatically. Missing date or time parts default to 1900-01-01 00:00:00. For example, when using a DATETIME HOUR TO MINUTE with the value of "11:45", the Oracle DATE value will be "1900-01-01 11:45:00".

Informix DATETIME YEAR TO FRACTION(n) data is stored in Oracle TIMESTAMP columns. The TIMESTAMP data type can store up to 9 digits in the fractional part, and therefore can store all precisions of Informix DATETIME.

Important:
  • Most arithmetic expressions involving dates ( for example, to add or remove a number of days from a date ) will produce the same result with Oracle. But keep in mind that Oracle evaluates date arithmetic expressions to NUMBER ( days.fraction ) while Informix evaluates to INTEGER when only DATE values are used in the expression, or to INTERVAL values if at least one DATETIME is used in the expression.
  • Even if a configuration parameter exists to get the Informix behavior, avoid fetching date values into CHAR or VARCHAR, to bypass the DBDATE / ISO format conversion difference with Oracle.

Date/time SQL functions

Informix SQL provides a set of date/time functions that do not exist in Oracle, but can be easily implemented as user-defined SQL function in your Oracle database.

For example, to implement the YEAR() function, create the following user function:
CREATE OR REPLACE FUNCTION year( adate in date )
RETURN NUMBER
IS
   v_year NUMBER;
BEGIN
   v_year := to_number( to_char( adate, 'YYYY' ) );
   RETURN (v_year);
END YEAR;
/

Similar SQL functions can be created, based on the following correspondance table:

Table 2. Informix and Oracle date/time SQL functions
Informix Oracle 
today
 
trunc( sysdate )
 
current year to second
 
sysdate
 
sysdate
 
systimestamp
 
day( value )
 
to_number( to_char( value, 'dd' ) )
 
extend( dtvalue,
  first to last )
 
to_date(
  nvl(
    to_char( dtvalue, 'fmt-mask-1' ),
    '19000101000000' ),
 'YYYYMMDDHH24MISS' )
 
mdy( m, d, y )
 
to_date( to_char(m,'09')
      || to_char(d,'09')
      || to_char(y,'0009'), 'MMDDYYYY' )
 
month( value )
 
to_number( to_char( value, 'mm' ) )
 
weekday( value )
 
to_number( to_char( value, 'd' ) ) - 1
 
year( value )
 
to_number( to_char( value, 'yyyy' ) )
 
date( "string" | integer )
 No equivalent: Depends from DBDATE with Informix