## Synopsis

Use the `CREATE DOMAIN` statement to create a user-defined data type with optional constraints, such as range of valid values, `DEFAULT`, `NOT NULL`, and `CHECK`. Domains are useful to abstract data types with common constraints. For example, domain can be used to represent phone number columns that will require the same `CHECK` constraints on the syntax.

## Syntax

{{%ebnf%}}
  create_domain,
  domain_constraint
{{%/ebnf%}}

## Semantics

### *create_domain*

### CREATE DOMAIN *name*

Specify the name of the domain. An error is raised if `name` already exists in the specified database.

### AS *data_type*

Specify the underlying data type.

### DEFAULT *expression*

Set the default value for columns of the domain data type.

### *domain_constraint*

#### CONSTRAINT *constraint_name*

Specify the optional name for the constraint.

##### NOT NULL

Do not allow null values.

##### NULL

Allow null values (default).

##### CHECK ( *expression* )

Enforce a constraint that the values of the domain must satisfy and returns a Boolean value.

The key word VALUE should be used to refer to the value being tested. Expressions evaluating to TRUE or UNKNOWN succeed.

## Examples

```plpgsql
yugabyte=# CREATE DOMAIN phone_number AS TEXT CHECK(VALUE ~ '^\d{3}-\d{3}-\d{4}$');
```

```plpgsql
yugabyte=# CREATE TABLE person(first_name TEXT, last_name TEXT, phone_number phone_number);
```

## See also

- [`DROP DOMAIN`](../ddl_drop_domain)
- [`ALTER DOMAIN`](../ddl_alter_domain)