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PostgreSQL DOMAIN Data Type -1

  • CREATE DOMAIN statement creates a user-defined data type with a range, optional DEFAULT, NOT NULL and CHECK constraint.
  • CREATE DOMAIN creates a new domain. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values). The user who defines a domain becomes its owner.
  • If a schema name is given (for example, CREATE DOMAIN myschema.mydomain ...) then the domain is created in the specified schema. Otherwise it is created in the current schema. The domain name must be unique among the types and domains existing in its schema.
  • Domains are useful for abstracting common constraints on fields into a single location for maintenance. For example, several tables might contain email address columns, all requiring the same CHECK constraint to verify the address syntax. Define a domain rather than setting up each table's constraint individually.
  • To be able to create a domain, you must have USAGE privilege on the underlying type.
CREATE DOMAIN name [ AS ] data_type
    [ COLLATE collation ]
    [ DEFAULT expression ]
    [ constraint [ ... ] ]

where constraint is:

[ CONSTRAINT constraint_name ]
{ NOT NULL | NULL | CHECK (expression) } 
The name (optionally schema-qualified) of a domain to be created.
The underlying data type of the domain. This can include array specifiers.
An optional collation for the domain. If no collation is specified, the underlying data type's default collation is used. The underlying type must be collatable if COLLATE is specified.
DEFAULT expression
The DEFAULT clause specifies a default value for columns of the domain data type. The value is any variable-free expression (but subqueries are not allowed). The data type of the default expression must match the data type of the domain. If no default value is specified, then the default value is the null value.
The default expression will be used in any insert operation that does not specify a value for the column. If a default value is defined for a particular column, it overrides any default associated with the domain. In turn, the domain default overrides any default value associated with the underlying data type.
CONSTRAINT constraint_name
An optional name for a constraint. If not specified, the system generates a name.
Values of this domain are prevented from being null (but see notes below).
Values of this domain are allowed to be null. This is the default.
CHECK (expression)
CHECK clauses specify integrity constraints or tests which values of the domain must satisfy. Each constraint must be an expression producing a Boolean result. It should use the key word VALUE to refer to the value being tested. Expressions evaluating to TRUE or UNKNOWN succeed. If the expression produces a FALSE result, an error is reported and the value is not allowed to be converted to the domain type.

When a domain has multiple CHECK constraints, they will be tested in alphabetical order by name. (PostgreSQL versions before 9.5 did not honor any particular firing order for CHECK constraints.)
Domain constraints, particularly NOT NULL, are checked when converting a value to the domain type. It is possible for a column that is nominally of the domain type to read as null despite there being such a constraint. For example, this can happen in an outer-join query, if the domain column is on the nullable side of the outer join. A more subtle example is
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
The empty scalar sub-SELECT will produce a null value that is considered to be of the domain type, so no further constraint checking is applied to it, and the insertion will succeed.
It is very difficult to avoid such problems, because of SQL's general assumption that a null value is a valid value of every data type. Best practice therefore is to design a domain's constraints so that a null value is allowed, and then to apply column NOT NULL constraints to columns of the domain type as needed, rather than directly to the domain type. 

postgres=# CREATE DOMAIN postal_code AS TEXT 
             CHECK(VALUE ~ '^\d{5}$'OR VALUE ~ '^\d{5}-\d{4}$');

postgres=# \dD
                                              List of domains
 Schema |    Name     | Type | Modifier |                              Check                               
 public | postal_code | text |          | CHECK (VALUE ~ '^\d{5}$'::text OR VALUE ~ '^\d{5}-\d{4}$'::text)
(1 row)
postgres=# CREATE TABLE us_address (address_id SERIAL PRIMARY KEY,
                                     street1 TEXT NOT NULL,
                                     street2 TEXT,
                                     street3 TEXT,
                                     city TEXT NOT NULL,
                                     postal postal_code NOT NULL);

postgres=# \d us_address
                                 Table "public.us_address"
   Column   |    Type     |                            Modifiers                            
 address_id | integer     | not null default nextval('us_address_address_id_seq'::regclass)
 street1    | text        | not null
 street2    | text        | 
 street3    | text        | 
 city       | text        | not null
 postal     | postal_code | not null
    "us_address_pkey" PRIMARY KEY, btree (address_id)
Insert some data into "us_address" table:
postgres=# insert into us_address values(1,'add1','add2','add3','cal',76321);

postgres=# insert into us_address values(2,'add1','add2','add3','cal',16321);

postgres=# insert into us_address values(3,'add1','add2','add3','cal',10001);

Domain checking is faster than constraints:
because domain will be check faster than primary key  you will understand from below example
postgres=# insert into us_address values(4,'add1','add2','add3','cal',09001);
ERROR:  value for domain postal_code violates check constraint "postal_code_check"

postgres=# insert into us_address values(3,'add1','add2','add3','cal',09001);
ERROR:  value for domain postal_code violates check constraint "postal_code_check"
Note: we already inserted "address_id" 3 but this will not throw error fastly only throw faster domain(postal_code violates check constraint "postal_code_check") Error

--finally we fetch data From "us_address" table
postgres=# select * from us_address;
 address_id | street1 | street2 | street3 | city | postal 
          1 | add1    | add2    | add3    | cal  | 76321
          2 | add1    | add2    | add3    | cal  | 16321
          3 | add1    | add2    | add3    | cal  | 10001
(3 rows)


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