Domains are created using the CREATE DOMAIN statement. For a full description of the syntax, see CREATE DOMAIN statement.

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

CREATE DOMAIN street_address CHAR( 35 );

CREATE DATATYPE can be used as an alternative to CREATE DOMAIN, but is not recommended.

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. Unlike with 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
);

Domains can be dropped by their owner or by a user with DBA authority, using the DROP DOMAIN statement:

DROP DOMAIN street_address;

This statement can be executed only if the data type is not used in any table in the database. If you attempt to drop a domain that is in use, an error message appears.

Many of the attributes associated with columns, such as allowing NULL values, having a DEFAULT value, and so on, can be built into a domain. 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 SQL Anywhere sample 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 );

By default, a column created using the id data type does not allow NULLs, defaults to an auto-incremented value, and must hold a positive number. Any identifier could be used instead of col in the @col variable.

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