Simple Domains

The following statement creates a data type named street_address, which is a 35-character string:

CREATE DOMAIN street_address CHAR( 35 )

You can use CREATE DATATYPE as an alternative to CREATE DOMAIN, but this is not recommended, as CREATE DOMAIN is the syntax used in the ISO/ANSI SQL standard.

Resource authority is required to create data types. Once a data type is created, the user ID that executed the CREATE DOMAIN statement is the owner of that data type. Any user can use the data type, and unlike other database objects, the owner name is never used to prefix the data type name.

The street_address data type may be used in exactly the same way as any other data type when defining columns. For example, the following table with two columns has the second column as a street_address column:

CREATE TABLE twocol (id INT,
street street_address)

Owners or DBAs can drop domains by issuing a COMMIT and then using the DROP DOMAIN statement:

DROP DOMAIN street_address

You can carry out this statement only if no tables in the database are using data type.

Constraints and Defaults with User-Defined Data Types

Many of the attributes associated with columns, such as allowing NULL values, having a DEFAULT value, and so on, can be built into a user-defined data type. Any column that is defined on the data type automatically inherits the NULL setting, CHECK condition, and DEFAULT values. This allows uniformity to be built into columns with a similar meaning throughout a database.

For example, many primary key columns in the demo database are integer columns holding ID numbers. The following statement creates a data type that may be useful for such columns:

CREATE DOMAIN id INT
NOT NULL
DEFAULT AUTOINCREMENT
CHECK( @col > 0 )

Any column created using the data type id is not allowed to hold NULLs, defaults to an autoincremented value, and must hold a positive number. Any identifier could be used instead of col in the @col variable.

The attributes of the data type can be overridden if needed by explicitly providing attributes for the column. A column created on data type id with NULL values explicitly allowed does allow NULLs, regardless of the setting in the id data type.