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 SQL Language StructureCommon Language Elements 

Domain Attributes

A domain definition has required and optional attributes.The data type is a required attribute.Optional attributes include:

  • a default value

  • to allow or forbid NULL

  • CHECK constraints

  • character set (for character data types and text BLOB fields)

  • collation (for character data types)

Sample domain definition
CREATE DOMAIN BOOL3 AS SMALLINT
  CHECK (VALUE IS NULL OR VALUE IN (0, 1));
See also

Explicit Data Type Conversion for the description of differences in the data conversion mechanism when domains are specified for the TYPE OF and TYPE OF COLUMN modifiers.

Domain Override

While defining a column using a domain, it is possible to override some attributes inherited from the domain.[fblangref50-dtyp-tbl-domoverride] summarises the rules for domain override.

Table 1. Rules for Overriding Domain Attributes in Column Definition
Attribute Override? Comments

Data type

No

 

Default value

Yes

 

Text character set

Yes

It can also be used to restore the default database values for the column

Text collation

Yes

 

CHECK constraints

Yes

To add new conditions to the check, you can use the corresponding CHECK clauses in the CREATE and ALTER statements at the table level.

NOT NULL

No

Often it is better to leave domain nullable in its definition and decide whether to make it NOT NULL when using the domain to define columns.