The search condition that defines a check constraint cannot contain the following elements: user-defined routines, subqueries, aggregates, host variables, or rowids. In addition, the search condition cannot contain the following built-in functions: CURRENT, USER, SITENAME, DBSERVERNAME, or TODAY.
When you specify a date value in a search condition, make sure you specify four digits for the year, so that the DBCENTURY environment variable has no effect on the condition. When you specify a two-digit year, the DBCENTURY environment variable can produce unpredictable results if the condition depends on an abbreviated year value. For more information about DBCENTURY, see the IBM Informix Guide to SQL: Reference.
More generally, the database server saves the settings of environment variables from the time of creation of check constraints. If any of these settings are subsequently changed in a way that can affect the evaluation of a condition in a check constraint, the new settings are disregarded, and the original environment variable settings are used when the condition is evaluated.
With a BYTE or TEXT column, you can check for NULL or not-NULL values. This constraint is the only constraint allowed on a BYTE or TEXT column.
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