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.

SQLite

SQLite 3 does not have a native type for date/time storage, but you can use data/time type names and functions based on the string representation of dates and times. The date/time values are stored in the TEXT native type.

The date/time functions of SQLite are based on standard DATE (YYYY-MM-DD), TIME (hh:mm:ss) and TIMESTAMP (YYYY-MM-DD hh:mm:ss) concepts.

For maximum flexibility with other RDBMS SQL languages, SQLite allows you to define table columns with your own type names. You can for example use the SMALLDATETIME, SMALLTIME, TIME(N), DATETIME(N) type names.

Solution

Note: Since SQLite allows various data type names, the conversion rules define specific data/time type names such as SMALLTIME, TINYDATETIME, to map original Informix date/time types. This allows the SQLite ODI driver and the fgldbsch tool detect the exact date/time type of a column. When a CREATE TABLE statement in a BDL program uses

DATETIME HOUR TO
MINUTE

, it is mapped to a SMALLTIME by the ODI driver, and when extracting the database schema, fgldbsch can recognized SMALLTIME as a BDL / Informix DATETIME HOUR TO MINUTE column.

Use the following conversion rules to map Informix date/time types to SQLite date/time (pseudo) types:

Table 1. Informix data types and SQLite equivalents
Informix data type	SQLite (pseudo data type
`DATE`	`DATE`
`DATETIME HOUR TO MINUTE`	`SMALLTIME`
`DATETIME HOUR TO SECOND`	`TIME`
`DATETIME HOUR TO FRACTION(n)`	`TIME(n)`
`DATETIME YEAR TO DAY`	`TINYDATETIME`
`DATETIME YEAR TO MINUTE`	`SMALLDATETIME`
`DATETIME YEAR TO SECOND`	`DATETIME`
`DATETIME YEAR TO FRACTION(n)`	`DATETIME(n)`
`DATETIME q1 TO q2 (different from above)`	`TIMESTAMP`

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.

In SQL statements, CURRENT expressions are converted to SQLite strftime('%Y-%m-%d %H:%M:%S','now'). The SQLite 'now' option returns the current date/time in UTC, while the FGL runtime system CURRENT instruction returns the current local time. Both values can different. Always consider using SQL parameters with program variables assigned by the CURRENT instruction of Genero BDL, instead of using CURRENT instructions in SQL statements.