<!-- doc/src/sgml/syntax.sgml -->

<chapter id="sql-syntax">
 <title>SQL Syntax</title>

 <indexterm zone="sql-syntax">
  <primary>syntax</primary>
  <secondary>SQL</secondary>
 </indexterm>

 <para>
  This chapter describes the syntax of SQL.  It forms the foundation
  for understanding the following chapters which will go into detail
  about how SQL commands are applied to define and modify data.
 </para>

 <para>
  We also advise users who are already familiar with SQL to read this
  chapter carefully because it contains several rules and concepts that
  are implemented inconsistently among SQL databases or that are
  specific to <productname>PostgreSQL</productname>.
 </para>

 <sect1 id="sql-syntax-lexical">
  <title>Lexical Structure</title>

  <indexterm>
   <primary>token</primary>
  </indexterm>

  <para>
   SQL input consists of a sequence of
   <firstterm>commands</firstterm>.  A command is composed of a
   sequence of <firstterm>tokens</firstterm>, terminated by a
   semicolon (<quote>;</quote>).  The end of the input stream also
   terminates a command.  Which tokens are valid depends on the syntax
   of the particular command.
  </para>

  <para>
   A token can be a <firstterm>key word</firstterm>, an
   <firstterm>identifier</firstterm>, a <firstterm>quoted
   identifier</firstterm>, a <firstterm>literal</firstterm> (or
   constant), or a special character symbol.  Tokens are normally
   separated by whitespace (space, tab, newline), but need not be if
   there is no ambiguity (which is generally only the case if a
   special character is adjacent to some other token type).
  </para>

   <para>
    For example, the following is (syntactically) valid SQL input:
<programlisting>
SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
INSERT INTO MY_TABLE VALUES (3, 'hi there');
</programlisting>
    This is a sequence of three commands, one per line (although this
    is not required; more than one command can be on a line, and
    commands can usefully be split across lines).
   </para>

  <para>
   Additionally, <firstterm>comments</firstterm> can occur in SQL
   input.  They are not tokens, they are effectively equivalent to
   whitespace.
  </para>

  <para>
   The SQL syntax is not very consistent regarding what tokens
   identify commands and which are operands or parameters.  The first
   few tokens are generally the command name, so in the above example
   we would usually speak of a <quote>SELECT</quote>, an
   <quote>UPDATE</quote>, and an <quote>INSERT</quote> command.  But
   for instance the <command>UPDATE</command> command always requires
   a <token>SET</token> token to appear in a certain position, and
   this particular variation of <command>INSERT</command> also
   requires a <token>VALUES</token> in order to be complete.  The
   precise syntax rules for each command are described in <xref linkend="reference"/>.
  </para>

  <sect2 id="sql-syntax-identifiers">
   <title>Identifiers and Key Words</title>

   <indexterm 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