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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>5.7. Modifying Tables</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="ddl-system-columns.html" title="5.6. System Columns" /><link rel="next" href="ddl-priv.html" title="5.8. Privileges" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.7. Modifying Tables</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-system-columns.html" title="5.6. System Columns">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-priv.html" title="5.8. Privileges">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-ALTER"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.7. Modifying Tables <a href="#DDL-ALTER" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-ADDING-A-COLUMN">5.7.1. Adding a Column</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-REMOVING-A-COLUMN">5.7.2. Removing a Column</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-ADDING-A-CONSTRAINT">5.7.3. Adding a Constraint</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-REMOVING-A-CONSTRAINT">5.7.4. Removing a Constraint</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-COLUMN-DEFAULT">5.7.5. Changing a Column's Default Value</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-COLUMN-TYPE">5.7.6. Changing a Column's Data Type</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-RENAMING-COLUMN">5.7.7. Renaming a Column</a></span></dt><dt><span class="sect2"><a href="ddl-alter.html#DDL-ALTER-RENAMING-TABLE">5.7.8. Renaming a Table</a></span></dt></dl></div><a id="id-1.5.4.9.2" class="indexterm"></a><p>
   When you create a table and you realize that you made a mistake, or
   the requirements of the application change, you can drop the
   table and create it again.  But this is not a convenient option if
   the table is already filled with data, or if the table is
   referenced by other database objects (for instance a foreign key
   constraint).  Therefore <span class="productname">PostgreSQL</span>
   provides a family of commands to make modifications to existing
   tables.  Note that this is conceptually distinct from altering
   the data contained in the table: here we are interested in altering
   the definition, or structure, of the table.
  </p><p>
   You can:
   </p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; "><li class="listitem"><p>Add columns</p></li><li class="listitem"><p>Remove columns</p></li><li class="listitem"><p>Add constraints</p></li><li class="listitem"><p>Remove constraints</p></li><li class="listitem"><p>Change default values</p></li><li class="listitem"><p>Change column data types</p></li><li class="listitem"><p>Rename columns</p></li><li class="listitem"><p>Rename tables</p></li></ul></div><p>

   All these actions are performed using the
   <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a>
   command, whose reference page contains details beyond those given
   here.
  </p><div class="sect2" id="DDL-ALTER-ADDING-A-COLUMN"><div class="titlepage"><div><div><h3 class="title">5.7.1. Adding a Column <a href="#DDL-ALTER-ADDING-A-COLUMN" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.5.2" class="indexterm"></a><p>
    To add a column, use a command like:
</p><pre class="programlisting">
ALTER TABLE products ADD COLUMN description text;
</pre><p>
    The new column is initially filled with whatever default
    value is given (null if you don't specify a <code class="literal">DEFAULT</code> clause).
   </p><div class="tip"><h3 class="title">Tip</h3><p>
     Adding a column with a constant default value does not require each row of
     the table to be updated when the <code class="command">ALTER TABLE</code> statement
     is executed. Instead, the default value will be returned the next time
     the row is accessed, and applied when the table is rewritten, making
     the <code class="command">ALTER TABLE</code> very fast even on large tables.
    </p><p>
     If the default value is volatile (e.g., <code class="function">clock_timestamp()</code>)
     each row will need to be updated with the value calculated at the time
     <code class="command">ALTER TABLE</code> is executed. To avoid a potentially
     lengthy update operation, particularly if you intend to fill the column
     with mostly nondefault values anyway, it may be preferable to add the
     column with no default, insert the correct values using
     <code class="command">UPDATE</code>, and then add any desired default as described
     below.
    </p></div><p>
    You can also define constraints on the column at the same time,
    using the usual syntax:
</p><pre class="programlisting">
ALTER TABLE products ADD COLUMN description text CHECK (description &lt;&gt; '');
</pre><p>
    In fact all the options that can be applied to a column description
    in <code class="command">CREATE TABLE</code> can be used here.  Keep in mind however
    that the default value must satisfy the given constraints, or the
    <code class="literal">ADD</code> will fail.  Alternatively, you can add
    constraints later (see below) after you've filled in the new column
    correctly.
   </p></div><div class="sect2" id="DDL-ALTER-REMOVING-A-COLUMN"><div class="titlepage"><div><div><h3 class="title">5.7.2. Removing a Column <a href="#DDL-ALTER-REMOVING-A-COLUMN" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.6.2" class="indexterm"></a><p>
    To remove a column, use a command like:
</p><pre class="programlisting">
ALTER TABLE products DROP COLUMN description;
</pre><p>
    Whatever data was in the column disappears.  Table constraints involving
    the column are dropped, too.  However, if the column is referenced by a
    foreign key constraint of another table,
    <span class="productname">PostgreSQL</span> will not silently drop that
    constraint.  You can authorize dropping everything that depends on
    the column by adding <code class="literal">CASCADE</code>:
</p><pre class="programlisting">
ALTER TABLE products DROP COLUMN description CASCADE;
</pre><p>
    See <a class="xref" href="ddl-depend.html" title="5.15. Dependency Tracking">Section 5.15</a> for a description of the general
    mechanism behind this.
   </p></div><div class="sect2" id="DDL-ALTER-ADDING-A-CONSTRAINT"><div class="titlepage"><div><div><h3 class="title">5.7.3. Adding a Constraint <a href="#DDL-ALTER-ADDING-A-CONSTRAINT" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.7.2" class="indexterm"></a><p>
    To add a constraint, the table constraint syntax is used.  For example:
</p><pre class="programlisting">
ALTER TABLE products ADD CHECK (name &lt;&gt; '');
ALTER TABLE products ADD CONSTRAINT some_name UNIQUE (product_no);
ALTER TABLE products ADD FOREIGN KEY (product_group_id) REFERENCES product_groups;
</pre><p>
   </p><p>
    To add a not-null constraint, which is normally not written as a table
    constraint, this special syntax is available:
</p><pre class="programlisting">
ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;
</pre><p>
    This command silently does nothing if the column already has a
    not-null constraint.
   </p><p>
    The constraint will be checked immediately, so the table data must
    satisfy the constraint before it can be added.
   </p></div><div class="sect2" id="DDL-ALTER-REMOVING-A-CONSTRAINT"><div class="titlepage"><div><div><h3 class="title">5.7.4. Removing a Constraint <a href="#DDL-ALTER-REMOVING-A-CONSTRAINT" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.8.2" class="indexterm"></a><p>
    To remove a constraint you need to know its name.  If you gave it
    a name then that's easy.  Otherwise the system assigned a
    generated name, which you need to find out.  The
    <span class="application">psql</span> command <code class="literal">\d
    <em class="replaceable"><code>tablename</code></em></code> can be helpful
    here; other interfaces might also provide a way to inspect table
    details.  Then the command is:
</p><pre class="programlisting">
ALTER TABLE products DROP CONSTRAINT some_name;
</pre><p>
   </p><p>
    As with dropping a column, you need to add <code class="literal">CASCADE</code> if you
    want to drop a constraint that something else depends on.  An example
    is that a foreign key constraint depends on a unique or primary key
    constraint on the referenced column(s).
   </p><p>
    Simplified syntax is available to drop a not-null constraint:
</p><pre class="programlisting">
ALTER TABLE products ALTER COLUMN product_no DROP NOT NULL;
</pre><p>
    This mirrors the <code class="literal">SET NOT NULL</code> syntax for adding a
    not-null constraint.  This command will silently do nothing if the column
    does not have a not-null constraint.  (Recall that a column can have at
    most one not-null constraint, so it is never ambiguous which constraint
    this command acts on.)
   </p></div><div class="sect2" id="DDL-ALTER-COLUMN-DEFAULT"><div class="titlepage"><div><div><h3 class="title">5.7.5. Changing a Column's Default Value <a href="#DDL-ALTER-COLUMN-DEFAULT" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.9.2" class="indexterm"></a><p>
    To set a new default for a column, use a command like:
</p><pre class="programlisting">
ALTER TABLE products ALTER COLUMN price SET DEFAULT 7.77;
</pre><p>
    Note that this doesn't affect any existing rows in the table, it
    just changes the default for future <code class="command">INSERT</code> commands.
   </p><p>
    To remove any default value, use:
</p><pre class="programlisting">
ALTER TABLE products ALTER COLUMN price DROP DEFAULT;
</pre><p>
    This is effectively the same as setting the default to null.
    As a consequence, it is not an error
    to drop a default where one hadn't been defined, because the
    default is implicitly the null value.
   </p></div><div class="sect2" id="DDL-ALTER-COLUMN-TYPE"><div class="titlepage"><div><div><h3 class="title">5.7.6. Changing a Column's Data Type <a href="#DDL-ALTER-COLUMN-TYPE" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.10.2" class="indexterm"></a><p>
    To convert a column to a different data type, use a command like:
</p><pre class="programlisting">
ALTER TABLE products ALTER COLUMN price TYPE numeric(10,2);
</pre><p>
    This will succeed only if each existing entry in the column can be
    converted to the new type by an implicit cast.  If a more complex
    conversion is needed, you can add a <code class="literal">USING</code> clause that
    specifies how to compute the new values from the old.
   </p><p>
    <span class="productname">PostgreSQL</span> will attempt to convert the column's
    default value (if any) to the new type, as well as any constraints
    that involve the column.  But these conversions might fail, or might
    produce surprising results.  It's often best to drop any constraints
    on the column before altering its type, and then add back suitably
    modified constraints afterwards.
   </p></div><div class="sect2" id="DDL-ALTER-RENAMING-COLUMN"><div class="titlepage"><div><div><h3 class="title">5.7.7. Renaming a Column <a href="#DDL-ALTER-RENAMING-COLUMN" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.11.2" class="indexterm"></a><p>
    To rename a column:
</p><pre class="programlisting">
ALTER TABLE products RENAME COLUMN product_no TO product_number;
</pre><p>
   </p></div><div class="sect2" id="DDL-ALTER-RENAMING-TABLE"><div class="titlepage"><div><div><h3 class="title">5.7.8. Renaming a Table <a href="#DDL-ALTER-RENAMING-TABLE" class="id_link">#</a></h3></div></div></div><a id="id-1.5.4.9.12.2" class="indexterm"></a><p>
    To rename a table:
</p><pre class="programlisting">
ALTER TABLE products RENAME TO items;
</pre><p>
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