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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="DDL-ALTER"
>5.5. Modifying Tables</A
></H1
><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
></P
></P><UL
COMPACT="COMPACT"
><LI
><P
>Add columns</P
></LI
><LI
><P
>Remove columns</P
></LI
><LI
><P
>Add constraints</P
></LI
><LI
><P
>Remove constraints</P
></LI
><LI
><P
>Change default values</P
></LI
><LI
><P
>Change column data types</P
></LI
><LI
><P
>Rename columns</P
></LI
><LI
><P
>Rename tables</P
></LI
></UL
><P>
All these actions are performed using the
<A
HREF="sql-altertable.html"
>ALTER TABLE</A
>
command, whose reference page contains details beyond those given
here.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-ALTER-ADDING-A-COLUMN"
>5.5.1. Adding a Column</A
></H2
><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 <TT
CLASS="LITERAL"
>DEFAULT</TT
> clause).
</P
><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 <> '');</PRE
><P>
In fact all the options that can be applied to a column description
in <TT
CLASS="COMMAND"
>CREATE TABLE</TT
> can be used here. Keep in mind however
that the default value must satisfy the given constraints, or the
<TT
CLASS="LITERAL"
>ADD</TT
> will fail. Alternatively, you can add
constraints later (see below) after you've filled in the new column
correctly.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Adding a column with a default requires updating each row of the
table (to store the new column value). However, if no default is
specified, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is able to avoid
the physical update. So if you intend to fill the column with
mostly nondefault values, it's best to add the column with no default,
insert the correct values using <TT
CLASS="COMMAND"
>UPDATE</TT
>, and then add any
desired default as described below.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-ALTER-REMOVING-A-COLUMN"
>5.5.2. Removing a Column</A
></H2
><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 <TT
CLASS="LITERAL"
>CASCADE</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE products DROP COLUMN description CASCADE;</PRE
><P>
See <A
HREF="ddl-depend.html"
>Section 5.12</A
> for a description of the general
mechanism behind this.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-ALTER-ADDING-A-CONSTRAINT"
>5.5.3. Adding a Constraint</A
></H2
><P
> To add a constraint, the table constraint syntax is used. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE products ADD CHECK (name <> '');
ALTER TABLE products ADD CONSTRAINT some_name UNIQUE (product_no);
ALTER TABLE products ADD FOREIGN KEY (product_group_id) REFERENCES product_groups;</PRE
><P>
To add a not-null constraint, which cannot be written as a table
constraint, use this syntax:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;</PRE
><P>
</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"
><H2
CLASS="SECT2"
><A
NAME="DDL-ALTER-REMOVING-A-CONSTRAINT"
>5.5.4. Removing a Constraint</A
></H2
><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 <TT
CLASS="LITERAL"
>\d
<TT
CLASS="REPLACEABLE"
><I
>tablename</I
></TT
></TT
> 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>
(If you are dealing with a generated constraint name like <TT
CLASS="LITERAL"
>$2</TT
>,
don't forget that you'll need to double-quote it to make it a valid
identifier.)
</P
><P
> As with dropping a column, you need to add <TT
CLASS="LITERAL"
>CASCADE</TT
> 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
> This works the same for all constraint types except not-null
constraints. To drop a not null constraint use:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE products ALTER COLUMN product_no DROP NOT NULL;</PRE
><P>
(Recall that not-null constraints do not have names.)
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN2786"
>5.5.5. Changing a Column's Default Value</A
></H2
><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 <TT
CLASS="COMMAND"
>INSERT</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="AEN2796"
>5.5.6. Changing a Column's Data Type</A
></H2
><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 <TT
CLASS="LITERAL"
>USING</TT
> 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"
><H2
CLASS="SECT2"
><A
NAME="AEN2806"
>5.5.7. Renaming a Column</A
></H2
><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"
><H2
CLASS="SECT2"
><A
NAME="AEN2813"
>5.5.8. Renaming a Table</A
></H2
><P
> To rename a table:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE products RENAME TO items;</PRE
><P>
</P
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