Using the CHECK Constraint

Check constraints allow you to designate conditions that must be met before data can be assigned to a column during an INSERT or UPDATE statement. When a reject file does not exist and a row evaluates to false for any check constraint defined on a table during an insert or update, the database server returns an error. When there is a reject file and a row evaluates to false for a check constraint defined on the table, the error is reported in the reject file and the statement continues to execute.

Check constraints are defined with search conditions. The search condition cannot contain subqueries, aggregates, host variables, or SPL routines. In addition, it cannot include the built-in functions CURRENT, USER, SITENAME, DBSERVERNAME, or TODAY. When you define a check constraint at the column level, the only column that the check constraint can check against is the column itself. In other words, the check constraint cannot depend upon values in other columns of the table.

DATAFILES Clause

The DATAFILES clause specifies external files that are opened when you use external tables.

DATAFILES Clause:
 
|--DATAFILES---------------------------------------------------->
 
      .-,------------------------------------------------------------.
      V                                                              |
>--(----'--+-DISK-+--:--+-coserver_num---+--:--+-fixed_path-----+--'-+--)--|
           '-PIPE-'     '-coserver_group-'     '-formatted_path-'

Element	Description	Restrictions	Syntax
coserver_group	Coserver group that contains the external data	Must exist	Identifier
coserver_num	Numeric ID of coserver containing the external data	Must exist	Literal Number
fixed_path	Pathname for input or output files in the definition of the external table	Must exist	Must conform to operating-system rules.
formatted_path	Formatted pathname that uses pattern-matching characters	Must exist	Must conform to operating-system rules.

You can use cogroup names and coserver numbers when you describe the input or output files for the external table definition. You can identify the DATAFILES either by coserver number or by cogroup name. A coserver number contains only digits. A cogroup name is a valid identifier that begins with a letter but otherwise contains any combination of letters, digits, and underscore symbols.

If you use only some of the available coservers for reading or writing files, you can designate these coservers as a cogroup using onutil and then use the cogroup name, rather than explicitly specifying each coserver and file separately. Whenever you use all coservers to manage external files, you can use the predefined coserver_group.

For examples of the DATAFILES clause, see Examples.