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Postgres documentation
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<refentry id="SQL-ALTERTABLE">
 <refmeta>
  <refentrytitle id="sql-altertable-title">
   ALTER TABLE
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   ALTER TABLE
  </refname>
  <refpurpose>
   Modifies table properties
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable> [ * ]
    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable
    class="PARAMETER">type</replaceable>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET DEFAULT <replaceable
    class="PARAMETER">value</replaceable> | DROP DEFAULT }
ALTER TABLE <replaceable class="PARAMETER">table</replaceable> [ * ]
    RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable
    class="PARAMETER">newcolumn</replaceable>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
    RENAME TO <replaceable class="PARAMETER">newtable</replaceable>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
    ADD <replaceable class="PARAMETER">table constraint definition</replaceable>
  </synopsis>

  <refsect2 id="R2-SQL-ALTERTABLE-1">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER"> table </replaceable></term>
      <listitem>
       <para>
	The name of an existing table to alter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER"> column </replaceable></term>
      <listitem>
       <para>
	Name of a new or existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER"> type </replaceable></term>
      <listitem>
       <para>
	Type of the new column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER"> newcolumn </replaceable></term>
      <listitem>
       <para>
	New name for an existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER"> newtable </replaceable></term>
      <listitem>
       <para>
	New name for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER"> table constraint definition </replaceable></term>
      <listitem>
       <para>
	New table constraint for the table
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-ALTERTABLE-2">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>ALTER</computeroutput></term>
      <listitem>
       <para>
	Message returned from column or table renaming.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>ERROR</computeroutput></term>
      <listitem>
       <para>
	Message returned if table or column is not available.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-ALTERTABLE-1">
  <refsect1info>
   <date>1998-04-15</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   The <literal>ADD COLUMN</literal> form adds a new column to the table
   using the same syntax as <xref linkend="SQL-CREATETABLE"
   endterm="SQL-CREATETABLE-title">. The <literal>ALTER COLUMN</literal> form
   allows you to set or remove the default for the column. Note that defaults
   only apply to newly inserted rows.
   The <literal>RENAME</literal> clause causes the name of a table or column
   to change without changing any of the data contained in
   the affected table. Thus, the table or column will
   remain of the same type and size after this command is
   executed.
   The ADD <replaceable class="PARAMETER">table constraint definition</replaceable> clause 
   adds a new constraint to the table using the same syntax as <xref
   linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-title">. 
  </para>

  <para>
   You must own the table in order to change its schema.
  </para>

  <refsect2 id="R2-SQL-ALTERTABLE-3">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Notes
</title>
   <para>
    The keyword <literal>COLUMN</literal> is noise and can be omitted.
   </para>

   <para>
    <quote>*</quote> following a name of a table indicates that the statement
    should be run over that table and all tables below it in the
    inheritance hierarchy;
    by default, the attribute will not be added to or renamed in any of the subclasses.

    This should always be done when adding or modifying an attribute in a
    superclass. If it is not, queries on  the  inheritance  hierarchy
    such as

    <programlisting>
SELECT <replaceable>NewColumn</replaceable> FROM <replaceable>SuperClass</replaceable>*
    </programlisting>

    will not work because the subclasses will be missing an attribute
    found in the superclass.
   </para>

   <para>
    In the current implementation, default and constraint clauses for the
    new column will be ignored. You can use the <literal>SET DEFAULT</literal>
    form of <command>ALTER TABLE</command> to set the default later.
    (You will also have to update the already existing rows to the
    new default value, using <xref linkend="sql-update-title"
    endterm="sql-update-title">.)
   </para>

   <para>
    In the current implementation, only FOREIGN KEY constraints can
    be added to a table.  To create or remove a unique constraint, create 
    a unique index (see <xref linkend="SQL-CREATEINDEX" 
    endterm="SQL-CREATEINDEX-title">).  To add check constraints
    you need to recreate and reload the table, using other 
    parameters to the <xref linkend="SQL-CREATETABLE" 
    endterm="SQL-CREATETABLE-title"> command.
   </para>

   <para>
    You must own the class in order to change its schema.
    Renaming any  part  of  the schema of a system
    catalog is not permitted.
    The <citetitle>PostgreSQL User's Guide</citetitle> has further
    information on inheritance.
   </para>

   <para>
    Refer to <command>CREATE TABLE</command> for a further description
    of valid arguments.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-ALTERTABLE-2">
  <title>
   Usage
  </title>
  <para>
   To add a column of type VARCHAR to a table:
   <programlisting>
ALTER TABLE distributors ADD COLUMN address VARCHAR(30);
   </programlisting>
  </para>

  <para>
   To rename an existing column:
   <programlisting>
ALTER TABLE distributors RENAME COLUMN address TO city;
   </programlisting>
  </para>

  <para>
   To rename an existing table:
   <programlisting>
ALTER TABLE distributors RENAME TO suppliers;
   </programlisting>
  </para>

  <para> 
   To add a foreign key constraint to a table:
   <programlisting>
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses(address) MATCH FULL
   </programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-ALTERTABLE-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-ALTERTABLE-4">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>SQL92</title>
    <para>
     The <literal>ADD COLUMN</literal> form is compliant with the exception that
     it does not support defaults and constraints, as explained above.
     The <literal>ALTER COLUMN</literal> form is in full compliance.
    </para>

    <para>
     SQL92 specifies some additional capabilities for <command>ALTER TABLE</command>
     statement which are not yet directly supported by <productname>Postgres</productname>:

    <variablelist>
     <varlistentry>
      <term>
       <synopsis>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable> DROP CONSTRAINT <replaceable class="PARAMETER">constraint</replaceable> { RESTRICT | CASCADE }
      </synopsis>
     </term>
     <listitem>
      <para>
       Removes a table constraint (such as a check constraint,
       unique constraint, or foreign key constraint). To 
       remove a unique constraint, drop a unique index,
       To remove other kinds of constraints you need to recreate
       and reload the table, using other parameters to the
       <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-title">
       command.
      </para>
      <para>
       For example, to drop any constraints on a table <literal>distributors</literal>:
       <programlisting>
CREATE TABLE temp AS SELECT * FROM distributors;
DROP TABLE distributors;
CREATE TABLE distributors AS SELECT * FROM temp;
DROP TABLE temp;
       </programlisting>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <synopsis>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable> DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { RESTRICT | CASCADE }
      </synopsis>
     </term>
     <listitem>
      <para>
       Removes a column from a table.
       Currently, to remove an existing column the table must be
       recreated and reloaded:
       <programlisting>
CREATE TABLE temp AS SELECT did, city FROM distributors;    
DROP TABLE distributors;
CREATE TABLE distributors (
    did      DECIMAL(3)  DEFAULT 1,
    name     VARCHAR(40) NOT NULL,
);
INSERT INTO distributors SELECT * FROM temp;
DROP TABLE temp;
       </programlisting>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
   </para>

   <para>
    The clauses to rename columns and tables are <productname>Postgres</productname>
    extensions from SQL92.
   </para>
 
  </refsect2>
 </refsect1>
</refentry>

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