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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!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">3CREATE DOMAIN <em class="replaceable"><code>name</code></em> [ AS ] <em class="replaceable"><code>data_type</code></em>4    [ COLLATE <em class="replaceable"><code>collation</code></em> ]5    [ DEFAULT <em class="replaceable"><code>expression</code></em> ]6    [ <em class="replaceable"><code>constraint</code></em> [ ... ] ]7 8<span class="phrase">where <em class="replaceable"><code>constraint</code></em> is:</span>9 10[ CONSTRAINT <em class="replaceable"><code>constraint_name</code></em> ]11{ NOT NULL | NULL | CHECK (<em class="replaceable"><code>expression</code></em>) }12</pre></div><div class="refsect1" id="id-1.9.3.62.5"><h2>Description</h2><p>13   <code class="command">CREATE DOMAIN</code> creates a new domain.  A domain is14   essentially a data type with optional constraints (restrictions on15   the allowed set of values).16   The user who defines a domain becomes its owner.17  </p><p>18   If a schema name is given (for example, <code class="literal">CREATE DOMAIN19   myschema.mydomain ...</code>) then the domain is created in the20   specified schema.  Otherwise it is created in the current schema.21   The domain name must be unique among the types and domains existing22   in its schema.23  </p><p>24   Domains are useful for abstracting common constraints on fields into25   a single location for maintenance.  For example, several tables might26   contain email address columns, all requiring the same CHECK constraint27   to verify the address syntax.28   Define a domain rather than setting up each table's constraint29   individually.30  </p><p>31   To be able to create a domain, you must have <code class="literal">USAGE</code>32   privilege on the underlying type.33  </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>34        The name (optionally schema-qualified) of a domain to be created.35       </p></dd><dt><span class="term"><em class="replaceable"><code>data_type</code></em></span></dt><dd><p>36        The underlying data type of the domain. This can include array37        specifiers.38       </p></dd><dt><span class="term"><em class="replaceable"><code>collation</code></em></span></dt><dd><p>39        An optional collation for the domain.  If no collation is40        specified, the domain has the same collation behavior as its41        underlying data type.42        The underlying type must be collatable if <code class="literal">COLLATE</code>43        is specified.44       </p></dd><dt><span class="term"><code class="literal">DEFAULT <em class="replaceable"><code>expression</code></em></code></span></dt><dd><p>45        The <code class="literal">DEFAULT</code> clause specifies a default value for46        columns of the domain data type.  The value is any47        variable-free expression (but subqueries are not allowed).48        The data type of the default expression must match the data49        type of the domain.  If no default value is specified, then50        the default value is the null value.51       </p><p>52        The default expression will be used in any insert operation53        that does not specify a value for the column.  If a default54        value is defined for a particular column, it overrides any55        default associated with the domain.  In turn, the domain56        default overrides any default value associated with the57        underlying data type.58       </p></dd><dt><span class="term"><code class="literal">CONSTRAINT <em class="replaceable"><code>constraint_name</code></em></code></span></dt><dd><p>59        An optional name for a constraint.  If not specified,60        the system generates a name.61       </p></dd><dt><span class="term"><code class="literal">NOT NULL</code></span></dt><dd><p>62        Values of this domain are prevented from being null63        (but see notes below).64       </p></dd><dt><span class="term"><code class="literal">NULL</code></span></dt><dd><p>65        Values of this domain are allowed to be null.  This is the default.66       </p><p>67        This clause is only intended for compatibility with68        nonstandard SQL databases.  Its use is discouraged in new69        applications.70       </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 tests71      which values of the domain must satisfy.72      Each constraint must be an expression73      producing a Boolean result.  It should use the key word <code class="literal">VALUE</code>74      to refer to the value being tested.  Expressions evaluating75      to TRUE or UNKNOWN succeed.  If the expression produces a FALSE result,76      an error is reported and the value is not allowed to be converted77      to the domain type.78     </p><p>79      Currently, <code class="literal">CHECK</code> expressions cannot contain80      subqueries nor refer to variables other than <code class="literal">VALUE</code>.81     </p><p>82      When a domain has multiple <code class="literal">CHECK</code> constraints,83      they will be tested in alphabetical order by name.84      (<span class="productname">PostgreSQL</span> versions before 9.5 did not honor any85      particular firing order for <code class="literal">CHECK</code> constraints.)86     </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.62.7"><h2>Notes</h2><p>87   Domain constraints, particularly <code class="literal">NOT NULL</code>, are checked when88   converting a value to the domain type.  It is possible for a column that89   is nominally of the domain type to read as null despite there being such90   a constraint.  For example, this can happen in an outer-join query, if91   the domain column is on the nullable side of the outer join.  A more92   subtle example is93</p><pre class="programlisting">94INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));95</pre><p>96   The empty scalar sub-SELECT will produce a null value that is considered97   to be of the domain type, so no further constraint checking is applied98   to it, and the insertion will succeed.99  </p><p>100   It is very difficult to avoid such problems, because of SQL's general101   assumption that a null value is a valid value of every data type.  Best practice102   therefore is to design a domain's constraints so that a null value is allowed,103   and then to apply column <code class="literal">NOT NULL</code> constraints to columns of104   the domain type as needed, rather than directly to the domain type.105  </p><p>106   <span class="productname">PostgreSQL</span> assumes that107   <code class="literal">CHECK</code> constraints' conditions are immutable, that is,108   they will always give the same result for the same input value.  This109   assumption is what justifies examining <code class="literal">CHECK</code>110   constraints only when a value is first converted to be of a domain type,111   and not at other times.  (This is essentially the same as the treatment112   of table <code class="literal">CHECK</code> constraints, as described in113   <a class="xref" href="ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS" title="5.4.1. Check Constraints">Section 5.4.1</a>.)114  </p><p>115   An example of a common way to break this assumption is to reference a116   user-defined function in a <code class="literal">CHECK</code> expression, and then117   change the behavior of that118   function.  <span class="productname">PostgreSQL</span> does not disallow that,119   but it will not notice if there are stored values of the domain type that120   now violate the <code class="literal">CHECK</code> constraint. That would cause a121   subsequent database dump and restore to fail.  The recommended way to122   handle such a change is to drop the constraint (using <code class="command">ALTER123   DOMAIN</code>), adjust the function definition, and re-add the124   constraint, thereby rechecking it against stored data.125  </p><p>126   It's also good practice to ensure that domain <code class="literal">CHECK</code>127   expressions will not throw errors.128  </p></div><div class="refsect1" id="id-1.9.3.62.8"><h2>Examples</h2><p>129   This example creates the <code class="type">us_postal_code</code> data type and130   then uses the type in a table definition.  A regular expression test131   is used to verify that the value looks like a valid US postal code:132 133</p><pre class="programlisting">134CREATE DOMAIN us_postal_code AS TEXT135CHECK(136   VALUE ~ '^\d{5}$'137OR VALUE ~ '^\d{5}-\d{4}$'138);139 140CREATE TABLE us_snail_addy (141  address_id SERIAL PRIMARY KEY,142  street1 TEXT NOT NULL,143  street2 TEXT,144  street3 TEXT,145  city TEXT NOT NULL,146  postal us_postal_code NOT NULL147);148</pre></div><div class="refsect1" id="SQL-CREATEDOMAIN-COMPATIBILITY"><h2>Compatibility</h2><p>149   The command <code class="command">CREATE DOMAIN</code> conforms to the SQL150   standard.151  </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>