{ "mode": "man", "parameter": "CREATE_DOMAIN", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/CREATE_DOMAIN/7/json", "generated": "2026-07-05T23:45:41Z", "synopsis": "CREATE DOMAIN name [ AS ] datatype\n[ COLLATE collation ]\n[ DEFAULT expression ]\n[ constraint [ ... ] ]\nwhere constraint is:\n[ CONSTRAINT constraintname ]\n{ NOT NULL | NULL | CHECK (expression) }", "sections": { "NAME": { "content": "CREATEDOMAIN - define a new domain\n", "subsections": [] }, "SYNOPSIS": { "content": "CREATE DOMAIN name [ AS ] datatype\n[ COLLATE collation ]\n[ DEFAULT expression ]\n[ constraint [ ... ] ]\n\nwhere constraint is:\n\n[ CONSTRAINT constraintname ]\n{ NOT NULL | NULL | CHECK (expression) }\n", "subsections": [] }, "DESCRIPTION": { "content": "CREATE DOMAIN creates a new domain. A domain is essentially a data type with optional\nconstraints (restrictions on the allowed set of values). The user who defines a domain\nbecomes its owner.\n\nIf a schema name is given (for example, CREATE DOMAIN myschema.mydomain ...) then the domain\nis created in the specified schema. Otherwise it is created in the current schema. The domain\nname must be unique among the types and domains existing in its schema.\n\nDomains are useful for abstracting common constraints on fields into a single location for\nmaintenance. For example, several tables might contain email address columns, all requiring\nthe same CHECK constraint to verify the address syntax. Define a domain rather than setting\nup each table's constraint individually.\n\nTo be able to create a domain, you must have USAGE privilege on the underlying type.\n", "subsections": [] }, "PARAMETERS": { "content": "name\nThe name (optionally schema-qualified) of a domain to be created.\n\ndatatype\nThe underlying data type of the domain. This can include array specifiers.\n\ncollation\nAn optional collation for the domain. If no collation is specified, the domain has the\nsame collation behavior as its underlying data type. The underlying type must be\ncollatable if COLLATE is specified.\n\nDEFAULT expression\nThe DEFAULT clause specifies a default value for columns of the domain data type. The\nvalue is any variable-free expression (but subqueries are not allowed). The data type of\nthe default expression must match the data type of the domain. If no default value is\nspecified, then the default value is the null value.\n\nThe default expression will be used in any insert operation that does not specify a value\nfor the column. If a default value is defined for a particular column, it overrides any\ndefault associated with the domain. In turn, the domain default overrides any default\nvalue associated with the underlying data type.\n\nCONSTRAINT constraintname\nAn optional name for a constraint. If not specified, the system generates a name.\n\nNOT NULL\nValues of this domain are prevented from being null (but see notes below).\n\nNULL\nValues of this domain are allowed to be null. This is the default.\n\nThis clause is only intended for compatibility with nonstandard SQL databases. Its use is\ndiscouraged in new applications.\n\nCHECK (expression)\nCHECK clauses specify integrity constraints or tests which values of the domain must\nsatisfy. Each constraint must be an expression producing a Boolean result. It should use\nthe key word VALUE to refer to the value being tested. Expressions evaluating to TRUE or\nUNKNOWN succeed. If the expression produces a FALSE result, an error is reported and the\nvalue is not allowed to be converted to the domain type.\n\nCurrently, CHECK expressions cannot contain subqueries nor refer to variables other than\nVALUE.\n\nWhen a domain has multiple CHECK constraints, they will be tested in alphabetical order\nby name. (PostgreSQL versions before 9.5 did not honor any particular firing order for\nCHECK constraints.)\n", "subsections": [] }, "NOTES": { "content": "Domain constraints, particularly NOT NULL, are checked when converting a value to the domain\ntype. It is possible for a column that is nominally of the domain type to read as null\ndespite there being such a constraint. For example, this can happen in an outer-join query,\nif the domain column is on the nullable side of the outer join. A more subtle example is\n\nINSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));\n\nThe empty scalar sub-SELECT will produce a null value that is considered to be of the domain\ntype, so no further constraint checking is applied to it, and the insertion will succeed.\n\nIt is very difficult to avoid such problems, because of SQL's general assumption that a null\nvalue is a valid value of every data type. Best practice therefore is to design a domain's\nconstraints so that a null value is allowed, and then to apply column NOT NULL constraints to\ncolumns of the domain type as needed, rather than directly to the domain type.\n\nPostgreSQL assumes that CHECK constraints' conditions are immutable, that is, they will\nalways give the same result for the same input value. This assumption is what justifies\nexamining CHECK constraints only when a value is first converted to be of a domain type, and\nnot at other times. (This is essentially the same as the treatment of table CHECK\nconstraints, as described in Section 5.4.1.)\n\nAn example of a common way to break this assumption is to reference a user-defined function\nin a CHECK expression, and then change the behavior of that function. PostgreSQL does not\ndisallow that, but it will not notice if there are stored values of the domain type that now\nviolate the CHECK constraint. That would cause a subsequent database dump and restore to\nfail. The recommended way to handle such a change is to drop the constraint (using ALTER\nDOMAIN), adjust the function definition, and re-add the constraint, thereby rechecking it\nagainst stored data.\n", "subsections": [] }, "EXAMPLES": { "content": "This example creates the uspostalcode data type and then uses the type in a table\ndefinition. A regular expression test is used to verify that the value looks like a valid US\npostal code:\n\nCREATE DOMAIN uspostalcode AS TEXT\nCHECK(\nVALUE ~ '^\\d{5}$'\nOR VALUE ~ '^\\d{5}-\\d{4}$'\n);\n\nCREATE TABLE ussnailaddy (\naddressid SERIAL PRIMARY KEY,\nstreet1 TEXT NOT NULL,\nstreet2 TEXT,\nstreet3 TEXT,\ncity TEXT NOT NULL,\npostal uspostalcode NOT NULL\n);\n", "subsections": [] }, "COMPATIBILITY": { "content": "The command CREATE DOMAIN conforms to the SQL standard.\n", "subsections": [] }, "SEE ALSO": { "content": "ALTER DOMAIN (ALTERDOMAIN(7)), DROP DOMAIN (DROPDOMAIN(7))\n\n\n\nPostgreSQL 14.23 2026 CREATE DOMAIN(7)", "subsections": [] } }, "summary": "CREATEDOMAIN - define a new domain", "flags": [], "examples": [ "This example creates the uspostalcode data type and then uses the type in a table", "definition. A regular expression test is used to verify that the value looks like a valid US", "postal code:", "CREATE DOMAIN uspostalcode AS TEXT", "CHECK(", "VALUE ~ '^\\d{5}$'", "OR VALUE ~ '^\\d{5}-\\d{4}$'", ");", "CREATE TABLE ussnailaddy (", "addressid SERIAL PRIMARY KEY,", "street1 TEXT NOT NULL,", "street2 TEXT,", "street3 TEXT,", "city TEXT NOT NULL,", "postal uspostalcode NOT NULL", ");" ], "see_also": [ { "name": "ALTERDOMAIN", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/ALTERDOMAIN/7/json" }, { "name": "DROPDOMAIN", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/DROPDOMAIN/7/json" }, { "name": "DOMAIN", "section": "7", "url": "https://www.chedong.com/phpMan.php/man/DOMAIN/7/json" } ] }