<!--
doc/src/sgml/ref/create_view.sgml
PostgreSQL documentation
-->

<refentry id="sql-createview">
 <indexterm zone="sql-createview">
  <primary>CREATE VIEW</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE VIEW</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE VIEW</refname>
  <refpurpose>define a new view</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] [ TEMP | TEMPORARY ] [ RECURSIVE ] VIEW <replaceable class="parameter">name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
    [ WITH ( <replaceable class="parameter">view_option_name</replaceable> [= <replaceable class="parameter">view_option_value</replaceable>] [, ... ] ) ]
    AS <replaceable class="parameter">query</replaceable>
    [ WITH [ CASCADED | LOCAL ] CHECK OPTION ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE VIEW</command> defines a view of a query.  The view
   is not physically materialized. Instead, the query is run every time
   the view is referenced in a query.
  </para>

  <para>
   <command>CREATE OR REPLACE VIEW</command> is similar, but if a view
   of the same name already exists, it is replaced.  The new query must
   generate the same columns that were generated by the existing view query
   (that is, the same column names in the same order and with the same data
   types), but it may add additional columns to the end of the list.  The
   calculations giving rise to the output columns may be completely different.
  </para>

  <para>
   If a schema name is given (for example, <literal>CREATE VIEW
   myschema.myview ...</literal>) then the view is created in the specified
   schema.  Otherwise it is created in the current schema.  Temporary
   views exist in a special schema, so a schema name cannot be given
   when creating a temporary view. The name of the view must be
   distinct from the name of any other view, table, sequence, index or foreign table
   in the same schema.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>TEMPORARY</literal> or <literal>TEMP</literal></term>
    <listitem>
     <para>
      If specified, the view is created as a temporary view.
      Temporary views are automatically dropped at the end of the
      current session.  Existing
      permanent relations with the same name are not visible to the
      current session while the temporary view exists, unless they are
      referenced with schema-qualified names.
     </para>

     <para>
      If any of the tables referenced by the view are temporary,
      the view is created as a temporary view (whether
      <literal>TEMPORARY</literal> is specified or not).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RECURSIVE</literal>
      <indexterm zone="sql-createview">
       <primary>RECURSIVE</primary>
       <secondary>in views</secondary>
      </indexterm>
    </term>
    <listitem>
     <para>
      Creates a recursive view.  The syntax
<synopsis>
CREATE RECURSIVE VIEW [ <replaceable>schema</replaceable> . ] <replaceable>view_name</replaceable> (<replaceable>column_names</replaceable>) AS SELECT <replaceable>...</replaceable>;
</synopsis>
      is equivalent to
<synopsis>
CREATE VIEW [ <replaceable>schema</replaceable> . ] <replaceable>view_name</replaceable> AS WITH RECURSIVE <replaceable>view_name</replaceable> (<replaceable>column_names</replaceable>) AS (SELECT <replaceable>...</replaceable>) SELECT <replaceable>column_names</replaceable> FROM <replaceable>view_name</replaceable>;
</synopsis>
      A view column name list must be specified for a recursive view.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of a view to be created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      An optional list of names to be used for columns of the view.
      If not given, the column names are deduced from the query.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>WITH ( <replaceable class="parameter">view_option_name</replaceable> [= <replaceable class="parameter">view_option_value</replaceable>] [, ... ] )</literal></term>
    <listitem>
     <para>
      This clause specifies optional parameters for a view; the following
      parameters are supported:

      <variablelist>
       <varlistentry>
        <term><literal>check_option</literal> (<type>enum</type>)</term>
        <listitem>
         <para>
          This parameter may be either <literal>local</literal> or
          <literal>cascaded</literal>, and is equivalent to specifying
          <literal>WITH [ CASCADED | LOCAL ] CHECK OPTION</literal> (see below).
          This option can be changed on existing views using <xref
          linkend="sql-alterview"/>.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>security_barrier</literal> (<type>boolean</type>)</term>
        <listitem>
         <para>
          This should be used if the view is intended to provide row-level
          security.  See <xref linkend="rules-privileges"/> for full details.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">query</replaceable></term>
    <listitem>
     <para>
      A <xref linkend="sql-select"/> or
      <xref linkend="sql-values"/> command
      which will provide the columns and rows of the view.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>WITH [ CASCADED | LOCAL ] CHECK OPTION</literal>
      <indexterm zone="sql-createview">
       <primary>CHECK OPTION</primary>
      </indexterm>
      <indexterm zone="sql-createview">
       <primary>WITH CHECK OPTION</primary>
      </indexterm>
    </term>
    <listitem>
     <para>
      This option controls the behavior of automatically updatable views.  When
      this option is specified, <command>INSERT</command> and <command>UPDATE</command>
      commands on the view will be checked to ensure that new rows satisfy the
      view-defining condition (that is, the new rows are checked to ensure that
      they are visible through the view).  If they are not, the update will be
      rejected.  If the <literal>CHECK OPTION</literal> is not specified,
      <command>INSERT</command> and <command>UPDATE</command> commands on the view are
      allowed to create rows that are not visible through the view.  The
      following check options are supported:

      <variablelist>
       <varlistentry>
        <term><literal>LOCAL</literal></term>
        <listitem>
         <para>
          New rows are only checked against the conditions defined directly in
          the view itself.  Any conditions defined on underlying base views are
          not checked (unless they also specify the <literal>CHECK OPTION</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>CASCADED</literal></term>
        <listitem>
         <para>
          New rows are checked against the conditions of the view and all
          underlying base views.  If the <literal>CHECK OPTION</literal> is specified,
          and neither <literal>LOCAL</literal> nor <literal>CASCADED</literal> is specified,
          then <literal>CASCADED</literal> is assumed.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>

     <para>
      The <literal>CHECK OPTION</literal> may not be used with <literal>RECURSIVE</literal>
      views.
     </para>

     <para>
      Note that the <literal>CHECK OPTION</literal> is only supported on views that
      are automatically updatable, and do not have <literal>INSTEAD OF</literal>
      triggers or <literal>INSTEAD</literal> rules.  If an automatically updatable
      view is defined on top of a base view that has <literal>INSTEAD OF</literal>
      triggers, then the <literal>LOCAL CHECK OPTION</literal> may be used to check
      the conditions on the automatically updatable view, but the conditions
      on the base view with <literal>INSTEAD OF</literal> triggers will not be
      checked (a cascaded check option will not cascade down to a
      trigger-updatable view, and any check options defined directly on a
      trigger-updatable view will be ignored).  If the view or any of its base
      relations has an <literal>INSTEAD</literal> rule that causes the
      <command>INSERT</command> or <command>UPDATE</command> command to be rewritten, then
      all check options will be ignored in the rewritten query, including any
      checks from automatically updatable views defined on top of the relation
      with the <literal>INSTEAD</literal> rule.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

   <para>
    Use the <xref linkend="sql-dropview"/>
    statement to drop views.
   </para>

   <para>
    Be careful that the names and types of the view's columns will be
    assigned the way you want.  For example:
<programlisting>
CREATE VIEW vista AS SELECT 'Hello World';
</programlisting>
    is bad form because the column name defaults to <literal>?column?</literal>;
    also, the column data type defaults to <type>text</type>, which might not
    be what you wanted.  Better style for a string literal in a view's
    result is something like:
<programlisting>
CREATE VIEW vista AS SELECT text 'Hello World' AS hello;
</programlisting>
   </para>

   <para>
    Access to tables referenced in the view is determined by permissions of
    the view owner.  In some cases, this can be used to provide secure but
    restricted access to the underlying tables.  However, not all views are
    secure against tampering; see <xref linkend="rules-privileges"/> for
    details.  Functions called in the view are treated the same as if they had
    been called directly from the query using the view.  Therefore the user of
    a view must have permissions to call all functions used by the view.
   </para>

   <para>
    When <command>CREATE OR REPLACE VIEW</command> is used on an
    existing view, only the view's defining SELECT rule is changed.
    Other view properties, including ownership, permissions, and non-SELECT
    rules, remain unchanged.  You must own the view
    to replace it (this includes being a member of the owning role).
   </para>

  <refsect2 id="sql-createview-updatable-views">
   <title>Updatable Views</title>

   <indexterm zone="sql-createview-updatable-views">
    <primary>updatable views</primary>
   </indexterm>

   <para>
    Simple views are automatically updatable: the system will allow
    <command>INSERT</command>, <command>UPDATE</command> and <command>DELETE</command> statements
    to be used on the view in the same way as on a regular table.  A view is
    automatically updatable if it satisfies all of the following conditions:

    <itemizedlist>
     <listitem>
      <para>
       The view must have exactly one entry in its <literal>FROM</literal> list,
       which must be a table or another updatable view.
      </para>
     </listitem>

     <listitem>
      <para>
       The view definition must not contain <literal>WITH</literal>,
       <literal>DISTINCT</literal>, <literal>GROUP BY</literal>, <literal>HAVING</literal>,
       <literal>LIMIT</literal>, or <literal>OFFSET</literal> clauses at the top level.
      </para>
     </listitem>

     <listitem>
      <para>
       The view definition must not contain set operations (<literal>UNION</literal>,
       <literal>INTERSECT</literal> or <literal>EXCEPT</literal>) at the top level.
      </para>
     </listitem>

     <listitem>
      <para>
       The view's select list must not contain any aggregates, window functions
       or set-returning functions.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    An automatically updatable view may contain a mix of updatable and
    non-updatable columns.  A column is updatable if it is a simple reference
    to an updatable column of the underlying base relation; otherwise the
    column is read-only, and an error will be raised if an <command>INSERT</command>
    or <command>UPDATE</command> statement attempts to assign a value to it.
   </para>

   <para>
    If the view is automatically updatable the system will convert any
    <command>INSERT</command>, <command>UPDATE</command> or <command>DELETE</command> statement
    on the view into the corresponding statement on the underlying base
    relation.  <command>INSERT</command> statements that have an <literal>ON
    CONFLICT UPDATE</literal> clause are fully supported.
   </para>

   <para>
    If an automatically updatable view contains a <literal>WHERE</literal>
    condition, the condition restricts which rows of the base relation are
    available to be modified by <command>UPDATE</command> and <command>DELETE</command>
    statements on the view.  However, an <command>UPDATE</command> is allowed to
    change a row so that it no longer satisfies the <literal>WHERE</literal>
    condition, and thus is no longer visible through the view.  Similarly,
    an <command>INSERT</command> command can potentially insert base-relation rows
    that do not satisfy the <literal>WHERE</literal> condition and thus are not
    visible through the view (<literal>ON CONFLICT UPDATE</literal> may
    similarly affect an existing row not visible through the view).
    The <literal>CHECK OPTION</literal> may be used to prevent
    <command>INSERT</command> and <command>UPDATE</command> commands from creating
    such rows that are not visible through the view.
   </para>

   <para>
    If an automatically updatable view is marked with the
    <literal>security_barrier</literal> property then all the view's <literal>WHERE</literal>
    conditions (and any conditions using operators which are marked as <literal>LEAKPROOF</literal>)
    will always be evaluated before any conditions that a user of the view has
    added.   See <xref linkend="rules-privileges"/> for full details.  Note that,
    due to this, rows which are not ultimately returned (because they do not
    pass the user's <literal>WHERE</literal> conditions) may still end up being locked.
    <command>EXPLAIN</command> can be used to see which conditions are
    applied at the relation level (and therefore do not lock rows) and which are
    not.
   </para>

   <para>
    A more complex view that does not satisfy all these conditions is
    read-only by default: the system will not allow an insert, update, or
    delete on the view.  You can get the effect of an updatable view by
    creating <literal>INSTEAD OF</literal> triggers on the view, which must
    convert attempted inserts, etc. on the view into appropriate actions
    on other tables.  For more information see <xref
    linkend="sql-createtrigger"/>.  Another possibility is to create rules
    (see <xref linkend="sql-createrule"/>), but in practice triggers are
    easier to understand and use correctly.
   </para>

   <para>
    Note that the user performing the insert, update or delete on the view
    must have the corresponding insert, update or delete privilege on the
    view.  In addition the view's owner must have the relevant privileges on
    the underlying base relations, but the user performing the update does
    not need any permissions on the underlying base relations (see
    <xref linkend="rules-privileges"/>).
   </para>
  </refsect2>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Create a view consisting of all comedy films:

<programlisting>
CREATE VIEW comedies AS
    SELECT *
    FROM films
    WHERE kind = 'Comedy';
</programlisting>
   This will create a view containing the columns that are in the
   <literal>film</literal> table at the time of view creation.  Though
   <literal>*</literal> was used to create the view, columns added later to
   the table will not be part of the view.
  </para>

  <para>
   Create a view with <literal>LOCAL CHECK OPTION</literal>:

<programlisting>
CREATE VIEW universal_comedies AS
    SELECT *
    FROM comedies
    WHERE classification = 'U'
    WITH LOCAL CHECK OPTION;
</programlisting>
   This will create a view based on the <literal>comedies</literal> view, showing
   only films with <literal>kind = 'Comedy'</literal> and
   <literal>classification = 'U'</literal>. Any attempt to <command>INSERT</command> or
   <command>UPDATE</command> a row in the view will be rejected if the new row
   doesn't have <literal>classification = 'U'</literal>, but the film
   <literal>kind</literal> will not be checked.
  </para>

  <para>
   Create a view with <literal>CASCADED CHECK OPTION</literal>:

<programlisting>
CREATE VIEW pg_comedies AS
    SELECT *
    FROM comedies
    WHERE classification = 'PG'
    WITH CASCADED CHECK OPTION;
</programlisting>
   This will create a view that checks both the <literal>kind</literal> and
   <literal>classification</literal> of new rows.
  </para>

  <para>
   Create a view with a mix of updatable and non-updatable columns:

<programlisting>
CREATE VIEW comedies AS
    SELECT f.*,
           country_code_to_name(f.country_code) AS country,
           (SELECT avg(r.rating)
            FROM user_ratings r
            WHERE r.film_id = f.id) AS avg_rating
    FROM films f
    WHERE f.kind = 'Comedy';
</programlisting>
   This view will support <command>INSERT</command>, <command>UPDATE</command> and
   <command>DELETE</command>.  All the columns from the <literal>films</literal> table will
   be updatable, whereas the computed columns <literal>country</literal> and
   <literal>avg_rating</literal> will be read-only.
  </para>

  <para>
   Create a recursive view consisting of the numbers from 1 to 100:
<programlisting>
CREATE RECURSIVE VIEW public.nums_1_100 (n) AS
    VALUES (1)
UNION ALL
    SELECT n+1 FROM nums_1_100 WHERE n &lt; 100;
</programlisting>
   Notice that although the recursive view's name is schema-qualified in this
   <command>CREATE</command>, its internal self-reference is not schema-qualified.
   This is because the implicitly-created CTE's name cannot be
   schema-qualified.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>CREATE OR REPLACE VIEW</command> is a
   <productname>PostgreSQL</productname> language extension.
   So is the concept of a temporary view.
   The <literal>WITH ( ... )</literal> clause is an extension as well.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterview"/></member>
   <member><xref linkend="sql-dropview"/></member>
   <member><xref linkend="sql-creatematerializedview"/></member>
  </simplelist>
 </refsect1>
</refentry>
