database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-11 20:28:21 +03:00
Code Activity

Fill in section on table modification.

Browse Source
This commit is contained in:
Peter Eisentraut
2002-09-05 21:32:23 +00:00
parent 497baca6b5
commit 04604fd1a4
1 changed files with 158 additions and 38 deletions
Download Patch File Download Diff File Expand all files Collapse all files

196
doc/src/sgml/ddl.sgml
View File

@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ddl.sgml,v 1.2 2002/08/28 20:17:44 momjian Exp $ --> <!-- $Header: /cvsroot/pgsql/doc/src/sgml/ddl.sgml,v 1.3 2002/09/05 21:32:23 petere Exp $ -->
<chapter id="ddl"> <chapter id="ddl">
<title>Data Definition</title> <title>Data Definition</title>
@ -151,7 +151,7 @@ DROP TABLE products;
columns will be filled with their respective default values. A columns will be filled with their respective default values. A
data manipulation command can also request explicitly that a column data manipulation command can also request explicitly that a column
be set to its default value, without knowing what this value is. be set to its default value, without knowing what this value is.
(Details about data manipulation commands are in the next chapter.) (Details about data manipulation commands are in <xref linkend="dml">.)
</para> </para>
<para> <para>
@ -263,7 +263,7 @@ CREATE TABLE products (
The first two constraints should look familiar. The third one The first two constraints should look familiar. The third one
uses a new syntax. It is not attached to a particular column, uses a new syntax. It is not attached to a particular column,
instead it appears as a separate item in the comma-separated instead it appears as a separate item in the comma-separated
column list. In general, column definitions and constraint column list. Column definitions and these constraint
definitions can be listed in mixed order. definitions can be listed in mixed order.
</para> </para>
@ -299,8 +299,10 @@ CREATE TABLE products (
<para> <para>
It should be noted that a check constraint is satisfied if the It should be noted that a check constraint is satisfied if the
check expression evaluates to true or the null value. To ensure check expression evaluates to true or the null value. Since most
that a column does not contain null values, the not-null expressions will evaluate to the null value if one operand is null
they will not prevent null values in the constrained columns. To
ensure that a column does not contain null values, the not-null
constraint described in the next section should be used. constraint described in the next section should be used.
</para> </para>
</sect2> </sect2>
@ -322,12 +324,13 @@ CREATE TABLE products (
<para> <para>
A not-null constraint is always written as a column constraint. A A not-null constraint is always written as a column constraint. A
not-null constraint is equivalent to creating a check constraint not-null constraint is functionally equivalent to creating a check
<literal>CHECK (<replaceable>column_name</replaceable> IS NOT constraint <literal>CHECK (<replaceable>column_name</replaceable>
NULL)</literal>, but in <productname>PostgreSQL</productname> IS NOT NULL)</literal>, but in
creating an explicit not-null constraint is more efficient. The <productname>PostgreSQL</productname> creating an explicit
drawback is that you cannot give explicit names to not-null not-null constraint is more efficient. The drawback is that you
constraints created that way. cannot give explicit names to not-null constraints created that
way.
</para> </para>
<para> <para>
@ -564,8 +567,8 @@ CREATE TABLE t1 (
<emphasis>FOREIGN KEY (b, c) REFERENCES other_table (c1, c2)</emphasis> <emphasis>FOREIGN KEY (b, c) REFERENCES other_table (c1, c2)</emphasis>
); );
</programlisting> </programlisting>
Of course, the number and type of constrained columns needs to Of course, the number and type of the constrained columns needs to
match the number and type of referenced columns. match the number and type of the referenced columns.
</para> </para>
<para> <para>
@ -847,13 +850,14 @@ SET SQL_Inheritance TO OFF;
<title>Modifying Tables</title> <title>Modifying Tables</title>
<para> <para>
When you create a table and you realize that you made a mistake, When you create a table and you realize that you made a mistake, or
then you can drop the table and create it again. But this is not a the requirements of the application changed, then you can drop the
convenient option if the table is already filled with data, or if table and create it again. But this is not a convenient option if
the table is referenced by other database objects (for instance a the table is already filled with data, or if the table is
foreign key constraint). Therefore referenced by other database objects (for instance a foreign key
<productname>PostgreSQL</productname> provides a family of commands constraint). Therefore <productname>PostgreSQL</productname>
to make modifications on existing tables. provides a family of commands to make modifications on existing
tables.
</para> </para>
<para> <para>
@ -862,6 +866,9 @@ SET SQL_Inheritance TO OFF;
<listitem> <listitem>
<para>Add columns,</para> <para>Add columns,</para>
</listitem> </listitem>
<listitem>
<para>Remove a column,</para>
</listitem>
<listitem> <listitem>
<para>Add constraints,</para> <para>Add constraints,</para>
</listitem> </listitem>
@ -879,22 +886,135 @@ SET SQL_Inheritance TO OFF;
</listitem> </listitem>
</itemizedlist> </itemizedlist>
In the current implementation you cannot All these actions are performed using the <literal>ALTER
<itemizedlist spacing="compact"> TABLE</literal> command.
<listitem>
<para>Remove a column,</para>
</listitem>
<listitem>
<para>Change the data type of a column.</para>
</listitem>
</itemizedlist>
These may be possible in a future release.
</para> </para>
<comment> <sect2>
OK, now explain how to do this. There's currently so much activity <title>Adding a Column</title>
on <literal>ALTER TABLE</literal> that I'm holding off a bit.
</comment> <para>
To add a column, use this command:
<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.
</para>
<para>
You can also define a constraint on the column at the same time,
using the usual syntax:
<programlisting>
ALTER TABLE products ADD COLUMN description text CHECK (description &lt;&gt; '');
</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.
</para>
</sect2>
<sect2>
<title>Removing a Column</title>
<para>
To remove a column, use this command:
<programlisting>
ALTER TABLE products DROP COLUMN description;
</programlisting>
</para>
</sect2>
<sect2>
<title>Adding a Constraint</title>
<para>
To add a constraint, the table constraint syntax is used. For example:
<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;
</programlisting>
To add a not-null constraint, which cannot be written as a table
constraint, use this syntax:
<programlisting>
ALTER TABLE products ALTER COLUMN product_no SET NOT NULL;
</programlisting>
</para>
<para>
The constraint will be checked immediately, so the table data must
satisfy the constraint before it can be added.
</para>
</sect2>
<sect2>
<title>Removing a Constraint</title>
<para>
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
<application>psql</application> command <literal>\d
<replaceable>tablename</replaceable></literal> can be helpful
here; other interfaces might also provide a way to inspect table
details. Then the command is:
<programlisting>
ALTER TABLE products DROP CONSTRAINT some_name;
</programlisting>
This works the same for all constraint types except not-null
constraints. To drop a not null constraint use
<programlisting>
ALTER TABLE products ALTER COLUMN product_no DROP NOT NULL;
</programlisting>
(Recall that not-null constraints do not have names.)
</para>
</sect2>
<sect2>
<title>Changing the Default</title>
<para>
To set a new default for a column, use a command like this:
<programlisting>
ALTER TABLE products ALTER COLUMN price SET DEFAULT 7.77;
</programlisting>
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
PostgreSQL. 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>
</sect2>
<sect2>
<title>Renaming a Column</title>
<para>
To rename a column:
<programlisting>
ALTER TABLE products RENAME COLUMN product_no TO product_number;
</programlisting>
</para>
</sect2>
<sect2>
<title>Renaming a Table</title>
<para>
To rename a table:
<programlisting>
ALTER TABLE products RENAME TO items;
</programlisting>
</para>
</sect2>
</sect1> </sect1>
<sect1 id="ddl-schemas"> <sect1 id="ddl-schemas">
@ -990,10 +1110,10 @@ DROP TABLE products CASCADE;
<note> <note>
<para> <para>
Foreign Key constraint dependencies and SERIAL dependencies from Foreign key constraint dependencies and serial column dependencies
<productname>PostgreSQL</productname> versions prior to 7.3 are from <productname>PostgreSQL</productname> versions prior to 7.3
<emphasis>not</emphasis> maintained or created during the upgrade are <emphasis>not</emphasis> maintained or created during the
process. However, all other dependency types are created successfully. upgrade process. All other dependency types survive the upgrade.
</para> </para>
</note> </note>
</sect1> </sect1>