DOMAIN is one of the object types in a relational database.A domain is created as a specific data type with attributes attached to it (think of attributes like length, precision or scale, nullability, check constraints).Once a domain has been defined in the database, it can be reused repeatedly to define table columns, PSQL arguments and PSQL local variables.Those objects inherit all attributes of the domain.Some attributes can be overridden when the new object is defined, if required.

This section describes the syntax of statements used to create, alter and drop domains.A detailed description of domains and their usage can be found in Custom Data Types — Domains.

`CREATE DOMAIN`

Creates a new domain

Available in

DSQL, ESQL

Syntax

CREATE DOMAIN name [AS] <datatype>
  [DEFAULT {<literal> | NULL | <context_var>}]
  [NOT NULL] [CHECK (<dom_condition>)]
  [COLLATE collation_name]

<datatype> ::=
  <scalar_datatype> | <blob_datatype> | <array_datatype>

<scalar_datatype> ::=
  !! See Scalar Data Types Syntax !!

<blob_datatype> ::=
  !! See BLOB Data Types Syntax !!

<array_datatype> ::=
  !! See Array Data Types Syntax !!

<dom_condition> ::=
    <val> <operator> <val>
  | <val> [NOT] BETWEEN <val> AND <val>
  | <val> [NOT] IN ({<val> [, <val> ...] | <select_list>})
  | <val> IS [NOT] NULL
  | <val> IS [NOT] DISTINCT FROM <val>
  | <val> [NOT] CONTAINING <val>
  | <val> [NOT] STARTING [WITH] <val>
  | <val> [NOT] LIKE <val> [ESCAPE <val>]
  | <val> [NOT] SIMILAR TO <val> [ESCAPE <val>]
  | <val> <operator> {ALL | SOME | ANY} (<select_list>)
  | [NOT] EXISTS (<select_expr>)
  | [NOT] SINGULAR (<select_expr>)
  | (<dom_condition>)
  | NOT <dom_condition>
  | <dom_condition> OR <dom_condition>
  | <dom_condition> AND <dom_condition>

<operator> ::=
    <> | != | ^= | ~= | = | < | > | <= | >=
  | !< | ^< | ~< | !> | ^> | ~>

<val> ::=
    VALUE
  | <literal>
  | <context_var>
  | <expression>
  | NULL
  | NEXT VALUE FOR genname
  | GEN_ID(genname, <val>)
  | CAST(<val> AS <cast_type>)
  | (<select_one>)
  | func([<val> [, <val> ...]])

<cast_type> ::= <domain_or_non_array_type> | <array_datatype>

<domain_or_non_array_type> ::=
  !! See Scalar Data Types Syntax !!

Table 1. `CREATE DOMAIN` Statement Parameters
Parameter	Description
name	Domain name.The maximum length is 63 characters
datatype	SQL data type
literal	A literal value that is compatible with datatype
context_var	Any context variable whose type is compatible with datatype
dom_condition	Domain condition
collation_name	Name of a collation that is valid for charset_name, if it is supplied with datatype or, otherwise, is valid for the default character set of the database
select_one	A scalar `SELECT` statement — selecting one column and returning only one row
select_list	A `SELECT` statement selecting one column and returning zero or more rows
select_expr	A `SELECT` statement selecting one or more columns and returning zero or more rows
expression	An expression resolving to a value that is compatible with datatype
genname	Sequence (generator) name
func	Internal function or UDF

The CREATE DOMAIN statement creates a new domain.

Any SQL data type can be specified as the domain type.

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Type-specific Details

Array Types

If the domain is to be an array, the base type can be any SQL data type except BLOB and array.
The dimensions of the array are specified between square brackets.
For each array dimension, one or two integer numbers define the lower and upper boundaries of its index range:
- By default, arrays are 1-based.The lower boundary is implicit and only the upper boundary need be specified.A single number smaller than 1 defines the range num..1 and a number greater than 1 defines the range 1..num.
- Two numbers separated by a colon (‘:’) and optional whitespace, the second greater than the first, can be used to define the range explicitly.One or both boundaries can be less than zero, as long as the upper boundary is greater than the lower.
When the array has multiple dimensions, the range definitions for each dimension must be separated by commas and optional whitespace.
Subscripts are validated only if an array actually exists.It means that no error messages regarding invalid subscripts will be returned if selecting a specific element returns nothing or if an array field is NULL.

String Types

You can use the CHARACTER SET clause to specify the character set for the CHAR, VARCHAR and BLOB (SUB_TYPE TEXT) types.If the character set is not specified, the character set specified as DEFAULT CHARACTER SET of the database will be used.If the database has no default character set, the character set NONE is applied by default when you create a character domain.

Warning

With character set NONE, character data are stored and retrieved the way they were submitted.Data in any encoding can be added to a column based on such a domain, but it is impossible to add this data to a column with a different encoding.Because no transliteration is performed between the source and destination encodings, errors may result.

DEFAULT Clause

The optional DEFAULT clause allows you to specify a default value for the domain.This value will be added to the table column that inherits this domain when the INSERT statement is executed, if no value is specified for it in the DML statement.Local variables and arguments in PSQL modules that reference this domain will be initialized with the default value.For the default value, use a literal of a compatible type or a context variable of a compatible type.

NOT NULL Constraint

Columns and variables based on a domain with the NOT NULL constraint will be prevented from being written as NULL, i.e. a value is required.

Caution

When creating a domain, take care to avoid specifying limitations that would contradict one another.For instance, NOT NULL and DEFAULT NULL are contradictory.

CHECK Constraint(s)

The optional CHECK clause specifies constraints for the domain.A domain constraint specifies conditions that must be satisfied by the values of table columns or variables that inherit from the domain.A condition must be enclosed in parentheses.A condition is a logical expression (also called a predicate) that can return the Boolean results TRUE, FALSE and UNKNOWN.A condition is considered satisfied if the predicate returns the value TRUE or “unknown value” (equivalent to NULL).If the predicate returns FALSE, the condition for acceptance is not met.

VALUE Keyword

The keyword VALUE in a domain constraint substitutes for the table column that is based on this domain or for a variable in a PSQL module.It contains the value assigned to the variable or the table column.VALUE can be used anywhere in the CHECK constraint, though it is usually used in the left part of the condition.

COLLATE

The optional COLLATE clause allows you to specify the collation if the domain is based on one of the string data types, including BLOBs with text subtypes.If no collation is specified, the collation will be the one that is default for the specified character set at the time the domain is created.

Who Can Create a Domain

The CREATE DOMAIN statement can be executed by:

Administrators
Users with the CREATE DOMAIN privilege

`CREATE DOMAIN` Examples

Creating a domain that can take values greater than 1,000, with a default value of 10,000.
```
CREATE DOMAIN CUSTNO AS
  INTEGER DEFAULT 10000
  CHECK (VALUE > 1000);
```
Creating a domain that can take the values 'Yes' and 'No' in the default character set specified during the creation of the database.
```
CREATE DOMAIN D_BOOLEAN AS
  CHAR(3) CHECK (VALUE IN ('Yes', 'No'));
```

Creating a domain with the UTF8 character set and the UNICODE_CI_AI collation.

CREATE DOMAIN FIRSTNAME AS
  VARCHAR(30) CHARACTER SET UTF8
  COLLATE UNICODE_CI_AI;

Creating a domain of the DATE type that will not accept NULL and uses the current date as the default value.
```
CREATE DOMAIN D_DATE AS
  DATE DEFAULT CURRENT_DATE
  NOT NULL;
```
Creating a domain defined as an array of 2 elements of the NUMERIC(18, 3) type.The starting array index is 1.
```
CREATE DOMAIN D_POINT AS
  NUMERIC(18, 3) [2];
```
Note

Domains defined over an array type may be used only to define table columns.You cannot use array domains to define local variables in PSQL modules.

Creating a domain whose elements can be only country codes defined in the COUNTRY table.

CREATE DOMAIN D_COUNTRYCODE AS CHAR(3)
  CHECK (EXISTS(SELECT * FROM COUNTRY
         WHERE COUNTRYCODE = VALUE));

Note	The example is given only to show the possibility of using predicates with queries in the domain test condition.It is not recommended to create this style of domain in practice unless the lookup table contains data that are never deleted.

`ALTER DOMAIN`

Alters the attributes of a domain or renames a domain

Available in

DSQL, ESQL

Syntax

ALTER DOMAIN domain_name
  [TO new_name]
  [TYPE <datatype>]
  [{SET DEFAULT {<literal> | NULL | <context_var>} | DROP DEFAULT}]
  [{SET | DROP} NOT NULL]
  [{ADD [CONSTRAINT] CHECK (<dom_condition>) | DROP CONSTRAINT}]

<datatype> ::=
   <scalar_datatype> | <blob_datatype>

<scalar_datatype> ::=
  !! See Scalar Data Types Syntax !!

<blob_datatype> ::=
  !! See BLOB Data Types Syntax !!

!! See also CREATE DOMAIN Syntax !!

Table 1. `ALTER DOMAIN` Statement Parameters
Parameter	Description
new_name	New name for domain.The maximum length is 63 characters
literal	A literal value that is compatible with datatype
context_var	Any context variable whose type is compatible with datatype

The ALTER DOMAIN statement enables changes to the current attributes of a domain, including its name.You can make any number of domain alterations in one ALTER DOMAIN statement.

`ALTER DOMAIN` clauses

TO name

Renames the domain, as long as there are no dependencies on the domain, i.e. table columns, local variables or procedure arguments referencing it.

SET DEFAULT

Sets a new default value for the domain, replacing any existing default.

DROP DEFAULT

Deletes a previously specified default value and replace it with NULL.

SET NOT NULL

Adds a NOT NULL constraint to the domain;columns or parameters of this domain will be prevented from being written as NULL, i.e. a value is required.

Note	Adding a `NOT NULL` constraint to an existing domain will subject all columns using this domain to a full data validation, so ensure that the columns have no nulls before attempting the change.

DROP NOT NULL

Drops the NOT NULL constraint from the domain.

Note	An explicit `NOT NULL` constraint on a column that depends on a domain prevails over the domain.In this situation, the modification of the domain to make it nullable does not propagate to the column.

ADD CONSTRAINT CHECK

Adds a CHECK constraint to the domain.If the domain already has a CHECK constraint, it has to be deleted first, using an ALTER DOMAIN statement that includes a DROP CONSTRAINT clause.

TYPE

Changes the data type of the domain to a different, compatible one.The system will forbid any change to the type that could result in data loss.An example would be if the number of characters in the new type were smaller than in the existing type.

Important

When you alter the attributes of a domain, existing PSQL code may become invalid.For information on how to detect it, read the piece entitled The RDB$VALID_BLR Field in Appendix A.

What `ALTER DOMAIN` Cannot Alter

If the domain was declared as an array, it is not possible to change its type or its dimensions;nor can any other type be changed to an array type.
The collation cannot be changed without dropping the domain and recreating it with the desired attributes.

Who Can Alter a Domain

The ALTER DOMAIN statement can be executed by:

Administrators
The owner of the domain
Users with the ALTER ANY DOMAIN privilege

Domain alterations can be prevented by dependencies from objects to which the user does not have sufficient privileges.

`ALTER DOMAIN` Examples

Changing the data type to INTEGER and setting or changing the default value to 2,000:
```
ALTER DOMAIN CUSTNO
  TYPE INTEGER
  SET DEFAULT 2000;
```
Renaming a domain.
```
ALTER DOMAIN D_BOOLEAN TO D_BOOL;
```

Deleting the default value and adding a constraint for the domain:

ALTER DOMAIN D_DATE
  DROP DEFAULT
  ADD CONSTRAINT CHECK (VALUE >= date '01.01.2000');

Changing the CHECK constraint:

ALTER DOMAIN D_DATE
  DROP CONSTRAINT;

ALTER DOMAIN D_DATE
  ADD CONSTRAINT CHECK
    (VALUE BETWEEN date '01.01.1900' AND date '31.12.2100');

Changing the data type to increase the permitted number of characters:
```
ALTER DOMAIN FIRSTNAME
  TYPE VARCHAR(50) CHARACTER SET UTF8;
```
Adding a NOT NULL constraint:
```
ALTER DOMAIN FIRSTNAME
  SET NOT NULL;
```
Removing a NOT NULL constraint:
```
ALTER DOMAIN FIRSTNAME
  DROP NOT NULL;
```

`DROP DOMAIN`

Drops an existing domain

Available in

DSQL, ESQL

Syntax

DROP DOMAIN domain_name

The DROP DOMAIN statement deletes a domain that exists in the database.It is not possible to delete a domain if it is referenced by any database table columns or used in any PSQL module.To delete a domain that is in use, all columns in all tables that refer to the domain have to be dropped and all references to the domain have to be removed from PSQL modules.

Who Can Drop a Domain

The DROP DOMAIN statement can be executed by:

Administrators
The owner of the domain
Users with the DROP ANY DOMAIN privilege

Example of `DROP DOMAIN`

Deleting the COUNTRYNAME domain

DROP DOMAIN COUNTRYNAME;

DOMAIN

DOMAIN

CREATE DOMAIN

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Type-specific Details

Who Can Create a Domain

CREATE DOMAIN Examples

ALTER DOMAIN

ALTER DOMAIN clauses

What ALTER DOMAIN Cannot Alter

Who Can Alter a Domain

ALTER DOMAIN Examples

DROP DOMAIN

Who Can Drop a Domain

Example of DROP DOMAIN

`DOMAIN`

`CREATE DOMAIN`

`CREATE DOMAIN` Examples

`ALTER DOMAIN`

`ALTER DOMAIN` clauses

What `ALTER DOMAIN` Cannot Alter

`ALTER DOMAIN` Examples

`DROP DOMAIN`

Example of `DROP DOMAIN`