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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE DOMAIN</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-createdatabase.html" title="CREATE DATABASE" /><link rel="next" href="sql-createeventtrigger.html" title="CREATE EVENT TRIGGER" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE DOMAIN</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createdatabase.html" title="CREATE DATABASE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createeventtrigger.html" title="CREATE EVENT TRIGGER">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-CREATEDOMAIN"><div class="titlepage"></div><a id="id-1.9.3.62.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE DOMAIN</span></h2><p>CREATE DOMAIN — define a new domain</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE DOMAIN <em class="replaceable"><code>name</code></em> [ AS ] <em class="replaceable"><code>data_type</code></em>
[ COLLATE <em class="replaceable"><code>collation</code></em> ]
[ DEFAULT <em class="replaceable"><code>expression</code></em> ]
[ <em class="replaceable"><code>constraint</code></em> [ ... ] ]
<span class="phrase">where <em class="replaceable"><code>constraint</code></em> is:</span>
[ CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> ]
{ NOT NULL | NULL | CHECK (<em class="replaceable"><code>expression</code></em>) }
</pre></div><div class="refsect1" id="id-1.9.3.62.5"><h2>Description</h2><p>
<code class="command">CREATE DOMAIN</code> 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.
</p><p>
If a schema name is given (for example, <code class="literal">CREATE DOMAIN
myschema.mydomain ...</code>) 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.
</p><p>
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.
</p><p>
To be able to create a domain, you must have <code class="literal">USAGE</code>
privilege on the underlying type.
</p></div><div class="refsect1" id="id-1.9.3.62.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>
The name (optionally schema-qualified) of a domain to be created.
</p></dd><dt><span class="term"><em class="replaceable"><code>data_type</code></em></span></dt><dd><p>
The underlying data type of the domain. This can include array
specifiers.
</p></dd><dt><span class="term"><em class="replaceable"><code>collation</code></em></span></dt><dd><p>
An optional collation for the domain. If no collation is
specified, the domain has the same collation behavior as its
underlying data type.
The underlying type must be collatable if <code class="literal">COLLATE</code>
is specified.
</p></dd><dt><span class="term"><code class="literal">DEFAULT <em class="replaceable"><code>expression</code></em></code></span></dt><dd><p>
The <code class="literal">DEFAULT</code> 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.
</p><p>
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.
</p></dd><dt><span class="term"><code class="literal">CONSTRAINT <em class="replaceable"><code>constraint_name</code></em></code></span></dt><dd><p>
An optional name for a constraint. If not specified,
the system generates a name.
</p></dd><dt><span class="term"><code class="literal">NOT NULL</code></span></dt><dd><p>
Values of this domain are prevented from being null
(but see notes below).
</p></dd><dt><span class="term"><code class="literal">NULL</code></span></dt><dd><p>
Values of this domain are allowed to be null. This is the default.
</p><p>
This clause is only intended for compatibility with
nonstandard SQL databases. Its use is discouraged in new
applications.
</p></dd><dt><span class="term"><code class="literal">CHECK (<em class="replaceable"><code>expression</code></em>)</code></span></dt><dd><p><code class="literal">CHECK</code> 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 <code class="literal">VALUE</code>
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.
</p><p>
Currently, <code class="literal">CHECK</code> expressions cannot contain
subqueries nor refer to variables other than <code class="literal">VALUE</code>.
</p><p>
When a domain has multiple <code class="literal">CHECK</code> constraints,
they will be tested in alphabetical order by name.
(<span class="productname">PostgreSQL</span> versions before 9.5 did not honor any
particular firing order for <code class="literal">CHECK</code> constraints.)
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.62.7"><h2>Notes</h2><p>
Domain constraints, particularly <code class="literal">NOT NULL</code>, 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
</p><pre class="programlisting">
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
</pre><p>
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.
</p><p>
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 <code class="literal">NOT NULL</code> constraints to columns of
the domain type as needed, rather than directly to the domain type.
</p><p>
<span class="productname">PostgreSQL</span> assumes that
<code class="literal">CHECK</code> constraints' conditions are immutable, that is,
they will always give the same result for the same input value. This
assumption is what justifies examining <code class="literal">CHECK</code>
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 <code class="literal">CHECK</code> constraints, as described in
<a class="xref" href="ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS" title="5.4.1. Check Constraints">Section 5.4.1</a>.)
</p><p>
An example of a common way to break this assumption is to reference a
user-defined function in a <code class="literal">CHECK</code> expression, and then
change the behavior of that
function. <span class="productname">PostgreSQL</span> does not disallow that,
but it will not notice if there are stored values of the domain type that
now violate the <code class="literal">CHECK</code> constraint. That would cause a
subsequent database dump and restore to fail. The recommended way to
handle such a change is to drop the constraint (using <code class="command">ALTER
DOMAIN</code>), adjust the function definition, and re-add the
constraint, thereby rechecking it against stored data.
</p><p>
It's also good practice to ensure that domain <code class="literal">CHECK</code>
expressions will not throw errors.
</p></div><div class="refsect1" id="id-1.9.3.62.8"><h2>Examples</h2><p>
This example creates the <code class="type">us_postal_code</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:
</p><pre class="programlisting">
CREATE DOMAIN us_postal_code AS TEXT
CHECK(
VALUE ~ '^\d{5}$'
OR VALUE ~ '^\d{5}-\d{4}$'
);
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
);
</pre></div><div class="refsect1" id="SQL-CREATEDOMAIN-COMPATIBILITY"><h2>Compatibility</h2><p>
The command <code class="command">CREATE DOMAIN</code> conforms to the SQL
standard.
</p></div><div class="refsect1" id="SQL-CREATEDOMAIN-SEE-ALSO"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-alterdomain.html" title="ALTER DOMAIN"><span class="refentrytitle">ALTER DOMAIN</span></a>, <a class="xref" href="sql-dropdomain.html" title="DROP DOMAIN"><span class="refentrytitle">DROP DOMAIN</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createdatabase.html" title="CREATE DATABASE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createeventtrigger.html" title="CREATE EVENT TRIGGER">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE DATABASE </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE EVENT TRIGGER</td></tr></table></div></body></html>
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