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><A
NAME="DDL-DEFAULT"
>5.2. Default Values</A
></H1
><A
NAME="AEN1966"
></A
><P
>   A column can be assigned a default value.  When a new row is
   created and no values are specified for some of the columns, those
   columns will be filled with their respective default values.  A
   data manipulation command can also request explicitly that a column
   be set to its default value, without having to know what that value is.
   (Details about data manipulation commands are in <A
HREF="dml.html"
>Chapter 6</A
>.)
  </P
><P
>   <A
NAME="AEN1971"
></A
>
   If no default value is declared explicitly, the default value is the
   null value.  This usually makes sense because a null value can
   be considered to represent unknown data.
  </P
><P
>   In a table definition, default values are listed after the column
   data type.  For example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE products (
    product_no integer,
    name text,
    price numeric <B
CLASS="EMPHASIS"
>DEFAULT 9.99</B
>
);</PRE
><P>
  </P
><P
>   The default value can be an expression, which will be
   evaluated whenever the default value is inserted
   (<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> when the table is created).  A common example
   is that a <TT
CLASS="TYPE"
>timestamp</TT
> column can have a default of <TT
CLASS="LITERAL"
>now()</TT
>,
   so that it gets set to the time of row insertion.  Another common
   example is generating a <SPAN
CLASS="QUOTE"
>"serial number"</SPAN
> for each row.
   In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> this is typically done by
   something like:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE products (
    product_no integer <B
CLASS="EMPHASIS"
>DEFAULT nextval('products_product_no_seq')</B
>,
    ...
);</PRE
><P>
   where the <TT
CLASS="LITERAL"
>nextval()</TT
> function supplies successive values
   from a <I
CLASS="FIRSTTERM"
>sequence object</I
> (see <A
HREF="functions-sequence.html"
>Section 9.15</A
>). This arrangement is sufficiently common
   that there's a special shorthand for it:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE products (
    product_no <B
CLASS="EMPHASIS"
>SERIAL</B
>,
    ...
);</PRE
><P>
   The <TT
CLASS="LITERAL"
>SERIAL</TT
> shorthand is discussed further in <A
HREF="datatype-numeric.html#DATATYPE-SERIAL"
>Section 8.1.4</A
>.
  </P
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