-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.36 2005/01/08 01:44:05 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.37 2005/01/09 17:47:30 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
<programlisting>
ALTER TABLE products ADD COLUMN description text;
</programlisting>
- The new column will initially be filled with null values in the
- existing rows of the table.
+ The new column is initially filled with whatever default
+ value is given (null if you don't specify a <literal>DEFAULT</> clause).
</para>
<para>
- You can also define a constraint on the column at the same time,
+ You can also define constraints on the column at the same time,
using the usual syntax:
<programlisting>
ALTER TABLE products ADD COLUMN description text CHECK (description <> '');
</programlisting>
- A new column cannot have a not-null constraint since the column
- initially has to contain null values. But you can add a not-null
- constraint later. Also, you cannot define a default value on a
- new column. According to the SQL standard, this would have to
- fill the new columns in the existing rows with the default value,
- which is not implemented yet. But you can adjust the column
- default later on.
+ In fact all the options that can be applied to a column description
+ in <command>CREATE TABLE</> can be used here. Keep in mind however
+ that the default value must satisfy the given constraints, or the
+ <literal>ADD</> will fail. Alternatively, you can add
+ constraints later (see below) after you've filled in the new column
+ correctly.
</para>
</sect2>
<programlisting>
ALTER TABLE products ALTER COLUMN price SET DEFAULT 7.77;
</programlisting>
+ Note that this doesn't affect any existing rows in the table, it
+ just changes the default for future <command>INSERT</> commands.
+ </para>
+
+ <para>
To remove any default value, use
<programlisting>
ALTER TABLE products ALTER COLUMN price DROP DEFAULT;
</programlisting>
- This is equivalent to setting the default to null, at least in
- <productname>PostgreSQL</>. As a consequence, it is not an error
+ This is equivalent to 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.
</para>