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37th chunk of `doc/src/sgml/ref/create_table.sgml`
1ce64dd08abb46965868cd06a51b6a464e1d86e2ae9670530000000100000a95
 reference columns of a unique index instead of
    columns of a primary key or unique constraint.
   </para>
  </refsect2>

  <refsect2>
   <title><literal>NULL</literal> <quote>Constraint</quote></title>

   <para>
    The <literal>NULL</literal> <quote>constraint</quote> (actually a
    non-constraint) is a <productname>PostgreSQL</productname>
    extension to the SQL standard that is included for compatibility with some
    other database systems (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>Constraint Naming</title>

   <para>
    The SQL standard says that table and domain constraints must have names
    that are unique across the schema containing the table or domain.
    <productname>PostgreSQL</productname> is laxer: it only requires
    constraint names to be unique across the constraints attached to a
    particular table or domain.  However, this extra freedom does not exist
    for index-based constraints (<literal>UNIQUE</literal>,
    <literal>PRIMARY KEY</literal>, and <literal>EXCLUDE</literal>
    constraints), because the associated index is named the same as the
    constraint, and index names must be unique across all relations within
    the same schema.
   </para>
  </refsect2>

  <refsect2>
   <title>Inheritance</title>

   <para>
    Multiple inheritance via the <literal>INHERITS</literal> clause is
    a <productname>PostgreSQL</productname> language extension.
    SQL:1999 and later define single inheritance using a
    different syntax and different semantics.  SQL:1999-style
    inheritance is not yet supported by
    <productname>PostgreSQL</productname>.
   </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();</literal>).  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</command>, so it seems cleaner to ignore this spec restriction.
   </para>
  </refsect2>

  <refsect2>
   <title>Multiple Identity Columns</title>

   <para>
    <productname>PostgreSQL</productname> allows a table to have more than one
    identity column.  The standard specifies that a table can have at most one
    identity column.  This is relaxed mainly to give more flexibility for
    doing schema changes or migrations.  Note that
    the <command>INSERT</command>
Title: CREATE TABLE Compatibility: NULL Constraint, Naming, Inheritance, and Zero-Column Tables
Summary
This section details further compatibility aspects of the CREATE TABLE command in PostgreSQL compared to the SQL standard. It highlights the NULL 'constraint' as a PostgreSQL extension, discusses the relaxed rules for constraint naming, explains the PostgreSQL extension of multiple inheritance, and notes the allowance of zero-column tables, which deviates from the SQL standard. It also briefly mentions the support for multiple identity columns.