zone="sql-syntax-identifiers">
    <primary>identifier</primary>
    <secondary>syntax of</secondary>
   </indexterm>

   <indexterm zone="sql-syntax-identifiers">
    <primary>name</primary>
    <secondary>syntax of</secondary>
   </indexterm>

   <indexterm zone="sql-syntax-identifiers">
    <primary>key word</primary>
    <secondary>syntax of</secondary>
   </indexterm>

   <para>
    Tokens such as <token>SELECT</token>, <token>UPDATE</token>, or
    <token>VALUES</token> in the example above are examples of
    <firstterm>key words</firstterm>, that is, words that have a fixed
    meaning in the SQL language.  The tokens <token>MY_TABLE</token>
    and <token>A</token> are examples of
    <firstterm>identifiers</firstterm>.  They identify names of
    tables, columns, or other database objects, depending on the
    command they are used in.  Therefore they are sometimes simply
    called <quote>names</quote>.  Key words and identifiers have the
    same lexical structure, meaning that one cannot know whether a
    token is an identifier or a key word without knowing the language.
    A complete list of key words can be found in <xref
    linkend="sql-keywords-appendix"/>.
   </para>

   <para>
    SQL identifiers and key words must begin with a letter
    (<literal>a</literal>-<literal>z</literal>, but also letters with
    diacritical marks and non-Latin letters) or an underscore
    (<literal>_</literal>).  Subsequent characters in an identifier or
    key word can be letters, underscores, digits
    (<literal>0</literal>-<literal>9</literal>), or dollar signs
    (<literal>$</literal>).  Note that dollar signs are not allowed in identifiers
    according to the letter of the SQL standard, so their use might render
    applications less portable.
    The SQL standard will not define a key word that contains
    digits or starts or ends with an underscore, so identifiers of this
    form are safe against possible conflict with future extensions of the
    standard.
   </para>

   <para>
    <indexterm><primary>identifier</primary><secondary>length</secondary></indexterm>
    The system uses no more than <symbol>NAMEDATALEN</symbol>-1
    bytes of an identifier; longer names can be written in
    commands, but they will be truncated.  By default,
    <symbol>NAMEDATALEN</symbol> is 64 so the maximum identifier
    length is 63 bytes. If this limit is problematic, it can be raised by
    changing the <symbol>NAMEDATALEN</symbol> constant in
    <filename>src/include/pg_config_manual.h</filename>.
   </para>

   <para>
    <indexterm>
     <primary>case sensitivity</primary>
     <secondary>of SQL commands</secondary>
    </indexterm>
    Key words and unquoted identifiers are case-insensitive.  Therefore:
<programlisting>
UPDATE MY_TABLE SET A = 5;
</programlisting>
    can equivalently be written as:
<programlisting>
uPDaTE my_TabLE SeT a = 5;
</programlisting>
    A convention often used is to write key words in upper
    case and names in lower case, e.g.:
<programlisting>
UPDATE my_table SET a = 5;
</programlisting>
   </para>

   <para>
    <indexterm>
     <primary>quotation marks</primary>
     <secondary>and identifiers</secondary>
    </indexterm>
    There is a second kind of identifier:  the <firstterm>delimited
    identifier</firstterm> or <firstterm>quoted
    identifier</firstterm>.  It is formed by enclosing an arbitrary
    sequence of characters in double-quotes
    (<literal>"</literal>). <!-- " font-lock mania --> A delimited
    identifier is always an identifier, never a key word.  So
    <literal>"select"</literal> could be used to refer to a column or
    table named <quote>select</quote>, whereas an unquoted
    <literal>select</literal> would be taken as a key word and
    would therefore provoke a parse error when used where a table or
    column name is expected.  The example can be written with quoted
    identifiers like this:
<programlisting>
UPDATE "my_table" SET "a" = 5;
</programlisting>