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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>8.18. Domain Types</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="rangetypes.html" title="8.17. Range Types" /><link rel="next" href="datatype-oid.html" title="8.19. Object Identifier Types" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">8.18. Domain Types</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="rangetypes.html" title="8.17. Range Types">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="datatype.html" title="Chapter 8. Data Types">Up</a></td><th width="60%" align="center">Chapter 8. Data Types</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="datatype-oid.html" title="8.19. Object Identifier Types">Next</a></td></tr></table><hr /></div><div class="sect1" id="DOMAINS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">8.18. Domain Types</h2></div></div></div><a id="id-1.5.7.26.2" class="indexterm"></a><a id="id-1.5.7.26.3" class="indexterm"></a><p>
A <em class="firstterm">domain</em> is a user-defined data type that is
based on another <em class="firstterm">underlying type</em>. Optionally,
it can have constraints that restrict its valid values to a subset of
what the underlying type would allow. Otherwise it behaves like the
underlying type — for example, any operator or function that
can be applied to the underlying type will work on the domain type.
The underlying type can be any built-in or user-defined base type,
enum type, array type, composite type, range type, or another domain.
</p><p>
For example, we could create a domain over integers that accepts only
positive integers:
</p><pre class="programlisting">
CREATE DOMAIN posint AS integer CHECK (VALUE > 0);
CREATE TABLE mytable (id posint);
INSERT INTO mytable VALUES(1); -- works
INSERT INTO mytable VALUES(-1); -- fails
</pre><p>
</p><p>
When an operator or function of the underlying type is applied to a
domain value, the domain is automatically down-cast to the underlying
type. Thus, for example, the result of <code class="literal">mytable.id - 1</code>
is considered to be of type <code class="type">integer</code> not <code class="type">posint</code>.
We could write <code class="literal">(mytable.id - 1)::posint</code> to cast the
result back to <code class="type">posint</code>, causing the domain's constraints
to be rechecked. In this case, that would result in an error if the
expression had been applied to an <code class="structfield">id</code> value of
1. Assigning a value of the underlying type to a field or variable of
the domain type is allowed without writing an explicit cast, but the
domain's constraints will be checked.
</p><p>
For additional information see <a class="xref" href="sql-createdomain.html" title="CREATE DOMAIN"><span class="refentrytitle">CREATE DOMAIN</span></a>.
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