<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_table.sgml,v 1.66 2003/04/14 18:08:58 tgl Exp $
PostgreSQL documentation
-->
<refentry id="SQL-CREATETABLE">
<refmeta>
<refentrytitle id="sql-createtable-title">CREATE TABLE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>CREATE TABLE</refname>
<refpurpose>define a new table</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
CREATE [ [ LOCAL ] { TEMPORARY | TEMP } ] TABLE <replaceable class="PARAMETER">table_name</replaceable> (
{ <replaceable class="PARAMETER">column_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ DEFAULT <replaceable>default_expr</> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [, ... ] ]
| <replaceable>table_constraint</replaceable> } [, ... ]
)
[ INHERITS ( <replaceable>parent_table</replaceable> [, ... ] ) ]
[ WITH OIDS | WITHOUT OIDS ]
[ ON COMMIT { PRESERVE ROWS | DELETE ROWS | DROP } ]
where <replaceable class="PARAMETER">column_constraint</replaceable> is:
[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
{ NOT NULL | NULL | UNIQUE | PRIMARY KEY |
CHECK (<replaceable class="PARAMETER">expression</replaceable>) |
REFERENCES <replaceable class="PARAMETER">reftable</replaceable> [ ( <replaceable class="PARAMETER">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL ]
[ ON DELETE <replaceable class="parameter">action</replaceable> ] [ ON UPDATE <replaceable class="parameter">action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
and <replaceable class="PARAMETER">table_constraint</replaceable> is:
[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
{ UNIQUE ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] ) |
PRIMARY KEY ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] ) |
CHECK ( <replaceable class="PARAMETER">expression</replaceable> ) |
FOREIGN KEY ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] ) REFERENCES <replaceable class="PARAMETER">reftable</replaceable> [ ( <replaceable class="PARAMETER">refcolumn</replaceable> [, ... ] ) ]
[ MATCH FULL | MATCH PARTIAL ] [ ON DELETE <replaceable class="parameter">action</replaceable> ] [ ON UPDATE <replaceable class="parameter">action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
</synopsis>
</refsynopsisdiv>
<refsect1 id="SQL-CREATETABLE-description">
<title>Description</title>
<para>
<command>CREATE TABLE</command> will create a new, initially empty table
in the current database. The table will be owned by the user issuing the
command.
</para>
<para>
If a schema name is given (for example, <literal>CREATE TABLE
myschema.mytable ...</>) then the table is created in the
specified schema. Otherwise it is created in the current schema (the one
at the front of the search path; see <literal>CURRENT_SCHEMA()</>).
TEMP tables exist in a special schema, so a schema name may not be
given when creating a TEMP table.
The table name must be distinct from the name of any other table,
sequence, index, or view in the same schema.
</para>
<para>
<command>CREATE TABLE</command> also automatically creates a data
type that represents the tuple type (structure type) corresponding
to one row of the table. Therefore, tables cannot have the same
name as any existing data type in the same schema.
</para>
<para>
A table cannot have more than 1600 columns. (In practice, the
effective limit is lower because of tuple-length constraints).
</para>
<para>
The optional constraint clauses specify constraints (or tests) that
new or updated rows must satisfy for an insert or update operation
to succeed. A constraint is a named rule: an SQL object which
helps define valid sets of values by putting limits on the results
of insert, update, or delete operations performed on a table.
</para>
<para>
There are two ways to define constraints: table constraints and
column constraints. A column constraint is defined as part of a
column definition. A table constraint definition is not tied to a
particular column, and it can encompass more than one column.
Every column constraint can also be written as a table constraint;
a column constraint is only a notational convenience if the
constraint only affects one column.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>[LOCAL] TEMPORARY</> or <literal>[LOCAL] TEMP</></term>
<listitem>
<para>
If specified, the table is created as a temporary table.
Temporary tables are automatically dropped at the end of a
session, or optionally at the end of the current transaction
(see ON COMMIT below). Existing permanent tables with the same
name are not visible to the current session while the temporary
table exists, unless they are referenced with schema-qualified
names. Any indexes created on a temporary table are automatically
temporary as well.
</para>
<para>
The <literal>LOCAL</literal> word is optional. But see under
<xref linkend="sql-createtable-compatibility"
endterm="sql-createtable-compatibility-title">.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">table_name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of the table to be created.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">column_name</replaceable></term>
<listitem>
<para>
The name of a column to be created in the new table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">data_type</replaceable></term>
<listitem>
<para>
The data type of the column. This may include array specifiers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DEFAULT
<replaceable>default_expr</replaceable></literal></term>
<listitem>
<para>
The <literal>DEFAULT</> clause assigns a default data value for
the column whose column definition it appears within. The value
is any variable-free expression (subselects and cross-references
to other columns in the current table are not allowed). The
data type of the default expression must match the data type of the
column.
</para>
<para>
The default expression will be used in any insert operation that
does not specify a value for the column. If there is no default
for a column, then the default is NULL.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>INHERITS ( <replaceable>parent_table</replaceable> [, ... ] )</literal></term>
<listitem>
<para>
The optional <literal>INHERITS</> clause specifies a list of
tables from which the new table automatically inherits all
columns. If the same column name exists in more than one parent
table, an error is reported unless the data types of the columns
match in each of the parent tables. If there is no conflict,
then the duplicate columns are merged to form a single column in
the new table. If the column name list of the new table
contains a column that is also inherited, the data type must
likewise match the inherited column(s), and the column
definitions are merged into one. However, inherited and new
column declarations of the same name need not specify identical
constraints: all constraints provided from any declaration are
merged together and all are applied to the new table. If the
new table explicitly specifies a default value for the column,
this default overrides any defaults from inherited declarations
of the column. Otherwise, any parents that specify default
values for the column must all specify the same default, or an
error will be reported.
</para>
<!--
<para>
<application>PostgreSQL</application> automatically allows the
created table to inherit
functions on tables above it in the inheritance hierarchy; that
is, if we create table <literal>foo</literal> inheriting from
<literal>bar</literal>, then functions that accept the tuple
type <literal>bar</literal> can also be applied to instances of
<literal>foo</literal>. (Currently, this works reliably for
functions on the first or only parent table, but not so well for
functions on additional parents.)
</para>
-->
</listitem>
</varlistentry>
<varlistentry>
<term><literal>WITH OIDS</> or <literal>WITHOUT OIDS</></term>
<listitem>
<para>
This optional clause specifies whether rows of the new table
should have OIDs (object identifiers) assigned to them. The
default is to have OIDs. (If the new table inherits from any
tables that have OIDs, then <literal>WITH OIDS</> is forced even
if the command says <literal>WITHOUT OIDS</>.)
</para>
<para>
Specifying <literal>WITHOUT OIDS</> allows the user to suppress
generation of OIDs for rows of a table. This may be worthwhile
for large tables, since it will reduce OID consumption and
thereby postpone wraparound of the 32-bit OID counter. Once the
counter wraps around, uniqueness of OIDs can no longer be
assumed, which considerably reduces their usefulness. Specifying
<literal>WITHOUT OIDS</literal> also reduces the space required
to store the table on disk by 4 bytes per row of the table,
thereby improving performance.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable></literal></term>
<listitem>
<para>
An optional name for a column or table constraint. If not specified,
the system generates a name.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>NOT NULL</></term>
<listitem>
<para>
The column is not allowed to contain NULL values.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>NULL</></term>
<listitem>
<para>
The column is allowed to contain NULL values. This is the default.
</para>
<para>
This clause is only available for compatibility with
non-standard SQL databases. Its use is discouraged in new
applications.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>UNIQUE</> (column constraint)</term>
<term><literal>UNIQUE ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] )</> (table constraint)</term>
<listitem>
<para>
The <literal>UNIQUE</literal> constraint specifies a rule that a
group of one or more distinct columns of a table may contain
only unique values. The behavior of the unique table constraint
is the same as that for column constraints, with the additional
capability to span multiple columns.
</para>
<para>
For the purpose of a unique constraint, NULL values are not
considered equal.
</para>
<para>
Each unique table constraint must name a set of columns that is
different from the set of columns named by any other unique or
primary key constraint defined for the table. (Otherwise it
would just be the same constraint listed twice.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PRIMARY KEY</> (column constraint)</term>
<term><literal>PRIMARY KEY ( <replaceable class="PARAMETER">column_name</replaceable> [, ... ] )</> (table constraint)</term>
<listitem>
<para>
The primary key constraint specifies that a column or columns of a table
may contain only unique (non-duplicate), non-NULL values.
Technically, <literal>PRIMARY KEY</literal> is merely a
combination of <literal>UNIQUE</> and <literal>NOT NULL</>, but
identifying a set of columns as primary key also provides
meta-data about the design of the schema, as a primary key
implies that other tables
may rely on this set of columns as a unique identifier for rows.
</para>
<para>
Only one primary key can be specified for a table, whether as a
column constraint or a table constraint.
</para>
<para>
The primary key constraint should name a set of columns that is
different from other sets of columns named by any unique
constraint defined for the same table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CHECK (<replaceable class="PARAMETER">expression</replaceable>)</literal></term>
<listitem>
<para>
<literal>CHECK</> clauses specify integrity constraints or tests
which new or updated rows must satisfy for an insert or update
operation to succeed. Each constraint must be an expression
producing a Boolean result. A condition appearing within a
column definition should reference that column's value only,
while a condition appearing as a table constraint may reference
multiple columns.
</para>
<para>
Currently, <literal>CHECK</literal> expressions cannot contain
subselects nor refer to variables other than columns of the
current row.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH <replaceable class="parameter">matchtype</replaceable> ] [ ON DELETE <replaceable class="parameter">action</replaceable> ] [ ON UPDATE <replaceable class="parameter">action</replaceable> ]</literal> (column constraint)</term>
<term><literal>FOREIGN KEY ( <replaceable class="parameter">column</replaceable> [, ... ] )
REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
[ MATCH <replaceable class="parameter">matchtype</replaceable> ]
[ ON DELETE <replaceable class="parameter">action</replaceable> ]
[ ON UPDATE <replaceable class="parameter">action</replaceable> ]</literal>
(table constraint)</term>
<listitem>
<para>
The <literal>REFERENCES</literal> column constraint specifies
that a group of one or more columns of the new table must only
contain values which match against values in the referenced
column(s) <replaceable class="parameter">refcolumn</replaceable>
of the referenced table <replaceable
class="parameter">reftable</replaceable>. If <replaceable
class="parameter">refcolumn</replaceable> is omitted, the
primary key of the <replaceable
class="parameter">reftable</replaceable> is used. The
referenced columns must be the columns of a unique or primary
key constraint in the referenced table.
</para>
<para>
A value added to these columns is matched against the values of
the referenced table and referenced columns using the given
match type. There are three match types: <literal>MATCH
FULL</>, <literal>MATCH PARTIAL</>, and a default match type if
none is specified. <literal>MATCH FULL</> will not allow one
column of a multicolumn foreign key to be NULL unless all
foreign key columns are NULL. The default match type allows some
foreign key columns to be NULL while other parts of the foreign
key are not NULL. <literal>MATCH PARTIAL</> is not yet
implemented.
</para>
<para>
In addition, when the data in the referenced columns is changed,
certain actions are performed on the data in this table's
columns. The <literal>ON DELETE</literal> clause specifies the
action to do when a referenced row in the referenced table is
being deleted. Likewise, the <literal>ON UPDATE</literal>
clause specifies the action to perform when a referenced column
in the referenced table is being updated to a new value. If the
row is updated, but the referenced column is not actually
changed, no action is done. There are the following possible
actions for each clause:
<variablelist>
<varlistentry>
<term><literal>NO ACTION</literal></term>
<listitem>
<para>
Produce an error indicating that the deletion or update
would create a foreign key constraint violation. This is
the default action.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>RESTRICT</literal></term>
<listitem>
<para>
Same as <literal>NO ACTION</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CASCADE</literal></term>
<listitem>
<para>
Delete any rows referencing the deleted row, or update the
value of the referencing column to the new value of the
referenced column, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SET NULL</literal></term>
<listitem>
<para>
Set the referencing column values to NULL.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SET DEFAULT</literal></term>
<listitem>
<para>
Set the referencing column values to their default value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
If primary key column is updated frequently, it may be wise to
add an index to the <literal>REFERENCES</literal> column so that
<literal>NO ACTION</literal> and <literal>CASCADE</literal>
actions associated with the <literal>REFERENCES</literal>
column can be more efficiently performed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DEFERRABLE</literal> or <literal>NOT DEFERRABLE</literal></term>
<listitem>
<para>
This controls whether the constraint can be deferred. A
constraint that is not deferrable will be checked immediately
after every command. Checking of constraints that are
deferrable may be postponed until the end of the transaction
(using the <xref linkend="sql-set-constraints" endterm="sql-set-constraints-title"> command).
<literal>NOT DEFERRABLE</literal> is the default. Only foreign
key constraints currently accept this clause. All other
constraint types are not deferrable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>INITIALLY IMMEDIATE</literal> or <literal>INITIALLY DEFERRED</literal></term>
<listitem>
<para>
If a constraint is deferrable, this clause specifies the default
time to check the constraint. If the constraint is
<literal>INITIALLY IMMEDIATE</literal>, it is checked after each
statement. This is the default. If the constraint is
<literal>INITIALLY DEFERRED</literal>, it is checked only at the
end of the transaction. The constraint check time can be
altered with the <xref linkend="sql-set-constraints" endterm="sql-set-constraints-title"> command.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ON COMMIT</literal></term>
<listitem>
<para>
The behavior of temporary tables at the end of a transaction
block can be controlled using <literal>ON COMMIT</literal>.
The three options are:
<variablelist>
<varlistentry>
<term><literal>PRESERVE ROWS</literal></term>
<listitem>
<para>
No special action is taken at the ends of transactions.
This is the default behavior.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DELETE ROWS</literal></term>
<listitem>
<para>
All rows in the temporary table will be deleted at the
end of each transaction block. Essentially, an automatic
<xref linkend="sql-truncate"> is done at each commit.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DROP</literal></term>
<listitem>
<para>
The temporary table will be dropped at the end of the current
transaction block.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 id="SQL-CREATETABLE-diagnostics">
<title>Diagnostics</title>
<msgset>
<msgentry>
<msg>
<msgmain>
<msgtext>
<simpara><computeroutput>CREATE TABLE</computeroutput></simpara>
</msgtext>
</msgmain>
</msg>
<msgexplan>
<para>
Message returned if table is successfully created.
</para>
</msgexplan>
</msgentry>
<msgentry>
<msg>
<msgmain>
<msgtext>
<simpara><computeroutput>ERROR</computeroutput></simpara>
</msgtext>
</msgmain>
</msg>
<msgexplan>
<para>
Message returned if table creation failed. This is usually
accompanied by some descriptive text, such as:
<computeroutput>ERROR: Relation '<replaceable
class="parameter">table</replaceable>' already
exists</computeroutput>, which occurs at run time if the table
specified already exists in the database.
</para>
</msgexplan>
</msgentry>
</msgset>
</refsect1>
<refsect1 id="SQL-CREATETABLE-notes">
<title>Notes</title>
<itemizedlist>
<listitem>
<para>
Whenever an application makes use of OIDs to identify specific
rows of a table, it is recommended to create a unique constraint
on the <structfield>oid</> column of that table, to ensure that
OIDs in the table will indeed uniquely identify rows even after
counter wraparound. Avoid assuming that OIDs are unique across
tables; if you need a database-wide unique identifier, use the
combination of <structfield>tableoid</> and row OID for the
purpose. (It is likely that future <productname>PostgreSQL</>
releases will use a separate OID counter for each table, so that
it will be <emphasis>necessary</>, not optional, to include
<structfield>tableoid</> to have a unique identifier
database-wide.)
</para>
<tip>
<para>
The use of <literal>WITHOUT OIDS</literal> is not recommended
for tables with no primary key, since without either an OID or a
unique data key, it is difficult to identify specific rows.
</para>
</tip>
</listitem>
<listitem>
<para>
<productname>PostgreSQL</productname> automatically creates an
index for each unique constraint and primary key constraint to
enforce the uniqueness. Thus, it is not necessary to create an
explicit index for primary key columns. (See <xref
linkend="sql-createindex" endterm="sql-createindex-title"> for more information.)
</para>
</listitem>
<listitem>
<para>
The SQL92 standard says that <literal>CHECK</> column constraints
may only refer to the column they apply to; only
<literal>CHECK</> table constraints may refer to multiple
columns. <productname>PostgreSQL</productname> does not enforce
this restriction; it treats column and table check constraints
alike.
</para>
</listitem>
<listitem>
<para>
Unique constraints and primary keys are not inherited in the
current implementation. This makes the combination of
inheritance and unique constraints rather dysfunctional.
</para>
</listitem>
</itemizedlist>
</refsect1>
<refsect1 id="SQL-CREATETABLE-examples">
<title>Examples</title>
<para>
Create table <structname>films</> and table
<structname>distributors</>:
<programlisting>
CREATE TABLE films (
code CHARACTER(5) CONSTRAINT firstkey PRIMARY KEY,
title CHARACTER VARYING(40) NOT NULL,
did DECIMAL(3) NOT NULL,
date_prod DATE,
kind CHAR(10),
len INTERVAL HOUR TO MINUTE
);
</programlisting>
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3) PRIMARY KEY DEFAULT NEXTVAL('serial'),
name VARCHAR(40) NOT NULL CHECK (name <> '')
);
</programlisting>
</para>
<para>
Create a table with a 2-dimensional array:
<programlisting>
CREATE TABLE array (
vector INT[][]
);
</programlisting>
</para>
<para>
Define a unique table constraint for the table films. Unique table
constraints can be defined on one or more columns of the table:
<programlisting>
CREATE TABLE films (
code CHAR(5),
title VARCHAR(40),
did DECIMAL(3),
date_prod DATE,
kind VARCHAR(10),
len INTERVAL HOUR TO MINUTE,
CONSTRAINT production UNIQUE(date_prod)
);
</programlisting>
</para>
<para>
Define a check column constraint:
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3) CHECK (did > 100),
name VARCHAR(40)
);
</programlisting>
</para>
<para>
Define a check table constraint:
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3),
name VARCHAR(40)
CONSTRAINT con1 CHECK (did > 100 AND name <> '')
);
</programlisting>
</para>
<para>
Define a primary key table constraint for the table
<structname>films</>. Primary key table constraints can be defined
on one or more columns of the table.
<programlisting>
CREATE TABLE films (
code CHAR(5),
title VARCHAR(40),
did DECIMAL(3),
date_prod DATE,
kind VARCHAR(10),
len INTERVAL HOUR TO MINUTE,
CONSTRAINT code_title PRIMARY KEY(code,title)
);
</programlisting>
</para>
<para>
Define a primary key constraint for table
<structname>distributors</>. The following two examples are
equivalent, the first using the table constraint syntax, the second
the column constraint notation.
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3),
name CHAR VARYING(40),
PRIMARY KEY(did)
);
</programlisting>
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3) PRIMARY KEY,
name VARCHAR(40)
);
</programlisting>
</para>
<para>
This assigns a literal constant default value for the column
<literal>name</literal>, and arranges for the default value of
column <literal>did</literal> to be generated by selecting the next
value of a sequence object. The default value of
<literal>modtime</literal> will be the time at which the row is
inserted.
<programlisting>
CREATE TABLE distributors (
name VARCHAR(40) DEFAULT 'luso films',
did INTEGER DEFAULT NEXTVAL('distributors_serial'),
modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
</programlisting>
</para>
<para>
Define two <literal>NOT NULL</> column constraints on the table
<classname>distributors</classname>, one of which is explicitly
given a name:
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3) CONSTRAINT no_null NOT NULL,
name VARCHAR(40) NOT NULL
);
</programlisting>
</para>
<para>
Define a unique constraint for the <literal>name</literal> column:
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3),
name VARCHAR(40) UNIQUE
);
</programlisting>
The above is equivalent to the following specified as a table constraint:
<programlisting>
CREATE TABLE distributors (
did DECIMAL(3),
name VARCHAR(40),
UNIQUE(name)
);
</programlisting>
</para>
</refsect1>
<refsect1 id="SQL-CREATETABLE-compatibility">
<title id="SQL-CREATETABLE-compatibility-title">Compatibility</title>
<para>
The <command>CREATE TABLE</command> command conforms to SQL92
and to a subset of SQL99, with exceptions listed below and in the
descriptions above.
</para>
<refsect2>
<title>Temporary Tables</title>
<para>
Although the syntax of <literal>CREATE TEMPORARY TABLE</literal>
resembles that of SQL92, the effect is not the same. In the standard,
temporary tables are defined just once and automatically exist (starting
with empty contents) in every session that needs them.
<productname>PostgreSQL</productname> instead
requires each session to issue its own <literal>CREATE TEMPORARY
TABLE</literal> command for each temporary table to be used. This allows
different sessions to use the same temporary table name for different
purposes, whereas the spec's approach constrains all instances of a
given temporary table name to have the same table structure.
</para>
<note>
<para>
The spec-mandated behavior of temporary tables is widely ignored.
<productname>PostgreSQL</productname>'s behavior on this point is similar
to that of several other RDBMSs.
</para>
</note>
<para>
SQL92's distinction between global and local temporary tables
is not in <productname>PostgreSQL</productname>, since that distinction
depends on the concept of modules, which
<productname>PostgreSQL</productname> does not have.
</para>
<para>
The <literal>ON COMMIT</literal> clause for temporary tables
also resembles SQL92, but has some differences.
If the <literal>ON COMMIT</> clause is omitted, SQL92 specifies that the
default behavior is <literal>ON COMMIT DELETE ROWS</>. However, the
default behavior in <productname>PostgreSQL</productname> is
<literal>ON COMMIT PRESERVE ROWS</literal>. The <literal>ON COMMIT
DROP</literal> option does not exist in SQL92.
</para>
</refsect2>
<refsect2>
<title><literal>NULL</literal> <quote>Constraint</quote></title>
<para>
The <literal>NULL</> <quote>constraint</quote> (actually a
non-constraint) is a <productname>PostgreSQL</productname>
extension to SQL92 that is included for compatibility with some
other RDBMSs (and for symmetry with the <literal>NOT
NULL</literal> constraint). Since it is the default for any
column, its presence is simply noise.
</para>
</refsect2>
<refsect2>
<title>Assertions</title>
<para>
An assertion is a special type of integrity constraint and shares
the same namespace as other constraints. However, an assertion is
not necessarily dependent on one particular table as constraints
are, so SQL92 provides the <command>CREATE ASSERTION</command>
statement as an alternate method for defining a constraint:
<synopsis>
CREATE ASSERTION <replaceable>name</replaceable> CHECK ( <replaceable>condition</replaceable> )
</synopsis>
</para>
<para>
<productname>PostgreSQL</> does not implement assertions at present.
</para>
</refsect2>
<!--
<para>
Domain constraints are defined by <command>CREATE
DOMAIN</command> or <command>ALTER DOMAIN</command> statements:
</para>
<para>
Domain constraint:
<synopsis>
[ CONSTRAINT <replaceable>constraint_name</replaceable> ] CHECK <replaceable>constraint</replaceable>
[ {INITIALLY DEFERRED | INITIALLY IMMEDIATE} ]
[ [ NOT ] DEFERRABLE ]
</synopsis>
</para>
-->
<refsect2>
<title>Inheritance</title>
<para>
Multiple inheritance via the <literal>INHERITS</literal> clause is
a <productname>PostgreSQL</productname> language extension. SQL99
(but not SQL92) defines single inheritance using a different
syntax and different semantics. SQL99-style inheritance is not
yet supported by <productname>PostgreSQL</productname>.
</para>
</refsect2>
<refsect2>
<title>Object IDs</title>
<para>
The <productname>PostgreSQL</productname> concept of OIDs is not
standard.
</para>
</refsect2>
<refsect2>
<title>Zero-column tables</title>
<para>
<productname>PostgreSQL</productname> allows a table of no columns
to be created (for example, <literal>CREATE TABLE foo();</>). This
is an extension from the SQL standard, which does not allow zero-column
tables. Zero-column tables are not in themselves very useful, but
disallowing them creates odd special cases for <command>ALTER TABLE
DROP COLUMN</>, so it seems cleaner to ignore this spec restriction.
</para>
</refsect2>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-altertable" endterm="sql-altertable-title"></member>
<member><xref linkend="sql-droptable" endterm="sql-droptable-title"></member>
</simplelist>
</refsect1>
</refentry>
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