'\" t .\" Title: CREATE DOMAIN .\" Author: The PostgreSQL Global Development Group .\" Generator: DocBook XSL Stylesheets vsnapshot .\" Date: 2021 .\" Manual: PostgreSQL 13.4 Documentation .\" Source: PostgreSQL 13.4 .\" Language: English .\" .TH "CREATE DOMAIN" "7" "2021" "PostgreSQL 13.4" "PostgreSQL 13.4 Documentation" .\" ----------------------------------------------------------------- .\" * Define some portability stuff .\" ----------------------------------------------------------------- .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .\" http://bugs.debian.org/507673 .\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .ie \n(.g .ds Aq \(aq .el .ds Aq ' .\" ----------------------------------------------------------------- .\" * set default formatting .\" ----------------------------------------------------------------- .\" disable hyphenation .nh .\" disable justification (adjust text to left margin only) .ad l .\" ----------------------------------------------------------------- .\" * MAIN CONTENT STARTS HERE * .\" ----------------------------------------------------------------- .SH "NAME" CREATE_DOMAIN \- define a new domain .SH "SYNOPSIS" .sp .nf CREATE DOMAIN \fIname\fR [ AS ] \fIdata_type\fR [ COLLATE \fIcollation\fR ] [ DEFAULT \fIexpression\fR ] [ \fIconstraint\fR [ \&.\&.\&. ] ] where \fIconstraint\fR is: [ CONSTRAINT \fIconstraint_name\fR ] { NOT NULL | NULL | CHECK (\fIexpression\fR) } .fi .SH "DESCRIPTION" .PP \fBCREATE DOMAIN\fR 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\&. .PP 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\&. .PP 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\*(Aqs constraint individually\&. .PP To be able to create a domain, you must have USAGE privilege on the underlying type\&. .SH "PARAMETERS" .PP \fIname\fR .RS 4 The name (optionally schema\-qualified) of a domain to be created\&. .RE .PP \fIdata_type\fR .RS 4 The underlying data type of the domain\&. This can include array specifiers\&. .RE .PP \fIcollation\fR .RS 4 An optional collation for the domain\&. If no collation is specified, the underlying data type\*(Aqs default collation is used\&. The underlying type must be collatable if COLLATE is specified\&. .RE .PP DEFAULT \fIexpression\fR .RS 4 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\&. .sp 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\&. .RE .PP CONSTRAINT \fIconstraint_name\fR .RS 4 An optional name for a constraint\&. If not specified, the system generates a name\&. .RE .PP NOT NULL .RS 4 Values of this domain are prevented from being null (but see notes below)\&. .RE .PP NULL .RS 4 Values of this domain are allowed to be null\&. This is the default\&. .sp This clause is only intended for compatibility with nonstandard SQL databases\&. Its use is discouraged in new applications\&. .RE .PP CHECK (\fIexpression\fR) .RS 4 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\&. .sp Currently, CHECK expressions cannot contain subqueries nor refer to variables other than VALUE\&. .sp 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\&.) .RE .SH "NOTES" .PP 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 .sp .if n \{\ .RS 4 .\} .nf INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false)); .fi .if n \{\ .RE .\} .sp 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\&. .PP It is very difficult to avoid such problems, because of SQL\*(Aqs general assumption that a null value is a valid value of every data type\&. Best practice therefore is to design a domain\*(Aqs 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\&. .PP PostgreSQL assumes that CHECK constraints\*(Aq conditions are immutable, that is, they will always give the same result for the same input value\&. This assumption is what justifies examining CHECK constraints only when a value is first converted to be of a domain type, and not at other times\&. (This is essentially the same as the treatment of table CHECK constraints, as described in Section\ \&5.4.1\&.) .PP An example of a common way to break this assumption is to reference a user\-defined function in a CHECK expression, and then change the behavior of that function\&. PostgreSQL does not disallow that, but it will not notice if there are stored values of the domain type that now violate the CHECK constraint\&. That would cause a subsequent database dump and reload to fail\&. The recommended way to handle such a change is to drop the constraint (using \fBALTER DOMAIN\fR), adjust the function definition, and re\-add the constraint, thereby rechecking it against stored data\&. .SH "EXAMPLES" .PP This example creates the us_postal_code 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: .sp .if n \{\ .RS 4 .\} .nf CREATE DOMAIN us_postal_code AS TEXT CHECK( VALUE ~ \*(Aq^\ed{5}$\*(Aq OR VALUE ~ \*(Aq^\ed{5}\-\ed{4}$\*(Aq ); CREATE TABLE us_snail_addy ( address_id SERIAL PRIMARY KEY, street1 TEXT NOT NULL, street2 TEXT, street3 TEXT, city TEXT NOT NULL, postal us_postal_code NOT NULL ); .fi .if n \{\ .RE .\} .SH "COMPATIBILITY" .PP The command \fBCREATE DOMAIN\fR conforms to the SQL standard\&. .SH "SEE ALSO" ALTER DOMAIN (\fBALTER_DOMAIN\fR(7)), DROP DOMAIN (\fBDROP_DOMAIN\fR(7))