3.15. Domains

Domains behave as data types with optional built-in constraints. The main advantage of using domains is to have in one place a data type with a specific validation instead of creating separate columns in tables and applying to them the needed constraints to provide correctness when assigning values.

One simple example is an email column in different tables: the data type, semantics, and validation for this column are the same; thus, a neat approach is to create a domain called email with the needed constraints and assign it as the data type of the columns that require it. This is the principle of object reuse: no redundancy and no repeated creation of the same object with the same purpose. Additionally, you have only one object to make the changes when needed.

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General
#	Field	Group box	Description
1	Type	Data type	The data type. Besides the built-in types, the list also holds the user-defined types, domains and other types of the model.
2	Length	Data type	Length, or number of characters or bits, for the types that accept one, like varchar(n) or bit(n). Disabled for the other types.
3	Precision	Data type	Precision, for the types that accept it, like numeric(p, s), or the number of fractional digits of seconds for time and timestamp. Disabled for the other types.
4	Dimension	Data type	Number of dimensions when the type is an array (type[]). 0 means that the type is not an array.
5	Format	Data type	Read-only preview of the data type as it is written in the SQL code, with length, precision, dimension and other modifiers applied.
6	Default Value		The default value of the domain. It must be written in the format accepted by the domain's data type.
7	Not null		Indicates whether the domain accepts null values. When checked, null values are rejected (NOT NULL).

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Check constraints
#	Field	Description
1	Name	The name of the check constraint being added to the domain.
2	Expression	The SQL expression of the check constraint, used to validate any value assigned to the domain.
3	Check constraints	The check constraints of the domain. Type the name and the expression in the fields above and use the add button to include a constraint.

References

Domain DDL
https://www.postgresql.org/docs/current/static/sql-createdomain.html