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><A
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>5.2. Default Values</A
></H1
><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
>
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 for a <TT
CLASS="TYPE"
>timestamp</TT
> column to have a default of <TT
CLASS="LITERAL"
>CURRENT_TIMESTAMP</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 9.16</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
>.
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