syntax.sgml

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<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 the 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 there are 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>

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

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

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

   <indexterm zone="sql-syntax-identifiers">
    <primary>key words</primary>
    <secondary>syntax</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, digits
    (<literal>0</literal>-<literal>9</literal>), or underscores,
    although the SQL standard will not define a key word that contains
    digits or starts or ends with an underscore.
   </para>

   <para>
    The system uses no more than <symbol>NAMEDATALEN</symbol>-1
    characters 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 (but at the time the system is built,
    <symbol>NAMEDATALEN</symbol> can be changed in
    <filename>src/include/postgres_ext.h</filename>).
   </para>

   <para>
    <indexterm>
     <primary>case sensitivity</primary>
     <secondary>SQL commands</secondary>
    </indexterm>
    Identifier and key word names 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>quotes</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>
   </para>

   <para>
    Quoted identifiers can contain any character other than a double
    quote itself.  This allows constructing table or column names that
    would otherwise not be possible, such as ones containing spaces or
    ampersands.  The length limitation still applies.
   </para>

   <para>
    Quoting an identifier also makes it case-sensitive, whereas
    unquoted names are always folded to lower case.  For example, the
    identifiers <literal>FOO</literal>, <literal>foo</literal> and
    <literal>"foo"</literal> are considered the same by
    <productname>PostgreSQL</productname>, but <literal>"Foo"</literal>
    and <literal>"FOO"</literal> are different from these three and
    each other.
    <footnote>
     <para>
      The folding of unquoted names to lower case in <productname>PostgreSQL</>
      is incompatible with the SQL standard, which says that unquoted
      names should be folded to upper case.  Thus, <literal>foo</literal>
      should be equivalent to <literal>"FOO"</literal> not
      <literal>"foo"</literal> according to the standard.  If you want to
      write portable applications you are advised to always quote a particular
      name or never quote it.
     </para>
    </footnote>
   </para>
  </sect2>


  <sect2 id="sql-syntax-constants">
   <title>Constants</title>

   <indexterm zone="sql-syntax-constants">
    <primary>constants</primary>
   </indexterm>

   <para>
    There are three kinds of <firstterm>implicitly-typed
    constants</firstterm> in <productname>PostgreSQL</productname>:
    strings, bit strings, and numbers.
    Constants can also be specified with explicit types, which can
    enable more accurate representation and more efficient handling by
    the system. The implicit constants are described below; explicit
    constants are discussed afterwards.
   </para>

   <sect3 id="sql-syntax-strings">
    <title>String Constants</title>

    <indexterm zone="sql-syntax-strings">
     <primary>character strings</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     <indexterm>
      <primary>quotes</primary>
      <secondary>escaping</secondary>
     </indexterm>
     A string constant in SQL is an arbitrary sequence of characters
     bounded by single quotes (<quote>'</quote>), e.g., <literal>'This
     is a string'</literal>.  SQL allows single quotes to be embedded
     in strings by typing two adjacent single quotes (e.g.,
     <literal>'Dianne''s horse'</literal>).  In
     <productname>PostgreSQL</productname> single quotes may
     alternatively be escaped with a backslash (<quote>\</quote>,
     e.g., <literal>'Dianne\'s horse'</literal>).
    </para>

    <para>
     C-style backslash escapes are also available:
     <literal>\b</literal> is a backspace, <literal>\f</literal> is a
     form feed, <literal>\n</literal> is a newline,
     <literal>\r</literal> is a carriage return, <literal>\t</literal>
     is a tab, and <literal>\<replaceable>xxx</replaceable></literal>,
     where <replaceable>xxx</replaceable> is an octal number, is the
     character with the corresponding ASCII code.  Any other character
     following a backslash is taken literally.  Thus, to include a
     backslash in a string constant, type two backslashes.
    </para>

    <para>
     The character with the code zero cannot be in a string constant.
    </para>

    <para>
     Two string constants that are only separated by whitespace
     <emphasis>with at least one newline</emphasis> are concatenated
     and effectively treated as if the string had been written in one
     constant.  For example:
<programlisting>
SELECT 'foo'
'bar';
</programlisting>
     is equivalent to
<programlisting>
SELECT 'foobar';
</programlisting>
     but
<programlisting>
SELECT 'foo'      'bar';
</programlisting>
     is not valid syntax.  (This slightly bizarre behavior is specified
     by <acronym>SQL9x</acronym>; <productname>PostgreSQL</productname> is
     following the standard.)
    </para>
   </sect3>

   <sect3 id="sql-syntax-bit-strings">
    <title>Bit-String Constants</title>

    <indexterm zone="sql-syntax-bit-strings">
     <primary>bit strings</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     Bit-string constants look like string constants with a
     <literal>B</literal> (upper or lower case) immediately before the
     opening quote (no intervening whitespace), e.g.,
     <literal>B'1001'</literal>.  The only characters allowed within
     bit-string constants are <literal>0</literal> and
     <literal>1</literal>.
    </para>

    <para>
     Alternatively, bit-string constants can be specified in hexadecimal
     notation, using a leading <literal>X</literal> (upper or lower case),
     e.g., <literal>X'1FF'</literal>.  This notation is equivalent to
     a bit-string constant with four binary digits for each hex digit.
    </para>

    <para>
     Both forms of bit-string constant can be continued
     across lines in the same way as regular string constants.
    </para>
   </sect3>

   <sect3>
    <title>Numeric Constants</title>

    <indexterm>
     <primary>numeric</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     Numeric constants are accepted in these general forms:
<synopsis>
<replaceable>digits</replaceable>
<replaceable>digits</replaceable>.<optional><replaceable>digits</replaceable></optional><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
<optional><replaceable>digits</replaceable></optional>.<replaceable>digits</replaceable><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
<replaceable>digits</replaceable>e<optional>+-</optional><replaceable>digits</replaceable>
</synopsis>
     where <replaceable>digits</replaceable> is one or more decimal
     digits (0 through 9).  At least one digit must be before or after the
     decimal point, if one is used.  At least one digit must follow the
     exponent marker (<literal>e</literal>), if one is present.
     There may not be any spaces or other characters embedded in the
     constant.  Notice that any leading plus or minus sign is not actually
     considered part of the constant; it is an operator applied to the
     constant.
    </para>

    <para>
     These are some examples of valid numeric constants:
<literallayout>
42
3.5
4.
.001
5e2
1.925e-3
</literallayout>
    </para>

    <para>
     A numeric constant that contains neither a decimal point nor an
     exponent is initially presumed to be type <type>integer</> if its
     value fits in type <type>integer</> (32 bits); otherwise it is
     presumed to be type <type>bigint</> if its
     value fits in type <type>bigint</> (64 bits); otherwise it is
     taken to be type <type>numeric</>.  Constants that contain	decimal
     points and/or exponents are always initially presumed to be type
     <type>numeric</>.
    </para>

    <para>
     The initially assigned data type of a numeric constant is just a
     starting point for the type resolution algorithms.  In most
     cases the constant will be automatically coerced to the most
     appropriate type depending on context.  When necessary, you
     can force a numeric value to be interpreted as a specific
     data type by casting it.  For example, you can force a numeric
     value to be treated as type <type>real</> (<type>float4</>)
     by writing

<programlisting>
REAL '1.23'  -- string style
1.23::REAL   -- PostgreSQL (historical) style
</programlisting>
     </para>
    </sect3>

   <sect3 id="sql-syntax-constants-generic">
    <title>Constants of Other Types</title>

    <indexterm>
     <primary>data types</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     A constant of an <emphasis>arbitrary</emphasis> type can be
     entered using any one of the following notations:
<synopsis>
<replaceable>type</replaceable> '<replaceable>string</replaceable>'
'<replaceable>string</replaceable>'::<replaceable>type</replaceable>
CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
</synopsis>
     The string's text is passed to the input conversion
     routine for the type called <replaceable>type</replaceable>. The
     result is a constant of the indicated type.  The explicit type
     cast may be omitted if there is no ambiguity as to the type the
     constant must be (for example, when it is passed as an argument
     to a non-overloaded function), in which case it is automatically
     coerced.
    </para>

    <para>
     It is also possible to specify a type coercion using a function-like
     syntax:
<synopsis>
<replaceable>typename</replaceable> ( '<replaceable>string</replaceable>' )
</synopsis>
     but not all type names may be used in this way; see <xref
     linkend="sql-syntax-type-casts"> for details.
    </para>

    <para>
     The <literal>::</literal>, <literal>CAST()</literal>, and
     function-call syntaxes can also be used to specify run-time type
     conversions of arbitrary expressions, as discussed in <xref
     linkend="sql-syntax-type-casts">.  But the form
     <replaceable>type</replaceable> '<replaceable>string</replaceable>'
     can only be used to specify the type of a literal constant.
     Another restriction on
     <replaceable>type</replaceable> '<replaceable>string</replaceable>'
     is that it does not work for array types; use <literal>::</literal>
     or <literal>CAST()</literal> to specify the type of an array constant.
    </para>
   </sect3>

   <sect3>
    <title>Array constants</title>

    <indexterm>
     <primary>arrays</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     The general format of an array constant is the following:
<synopsis>
'{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
</synopsis>
     where <replaceable>delim</replaceable> is the delimiter character
     for the type, as recorded in its <literal>pg_type</literal>
     entry.  (For all built-in types, this is the comma character
     <quote><literal>,</literal></>.)  Each <replaceable>val</replaceable> is either a constant
     of the array element type, or a subarray.  An example of an
     array constant is
<programlisting>
'{{1,2,3},{4,5,6},{7,8,9}}'
</programlisting>
     This constant is a two-dimensional, 3-by-3 array consisting of three
     subarrays of integers.
    </para>

    <para>
     Individual array elements can be placed between double-quote
     marks (<literal>"</literal>) <!-- " --> to avoid ambiguity
     problems with respect to whitespace.  Without quote marks, the
     array-value parser will skip leading whitespace.
    </para>

    <para>
     (Array constants are actually only a special case of the generic
     type constants discussed in the previous section.  The constant
     is initially treated as a string and passed to the array input
     conversion routine.  An explicit type specification might be
     necessary.)
    </para>
   </sect3>
  </sect2>


  <sect2 id="sql-syntax-operators">
   <title>Operators</title>

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

   <para>
    An operator is a sequence of up to <symbol>NAMEDATALEN</symbol>-1
    (63 by default) characters from the following list:
<literallayout>
+ - * / &lt; &gt; = ~ ! @ # % ^ &amp; | ` ? $
</literallayout>

    There are a few restrictions on operator names, however:
    <itemizedlist>
     <listitem>
      <para>
       <literal>$</> (dollar) cannot be a single-character operator, although it
       can be part of a multiple-character operator name.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>--</literal> and <literal>/*</literal> cannot appear
       anywhere in an operator name, since they will be taken as the
       start of a comment.
      </para>
     </listitem>

     <listitem>
      <para>
       A multiple-character operator name cannot end in <literal>+</> or <literal>-</>,
       unless the name also contains at least one of these characters:
<literallayout>
~ ! @ # % ^ &amp; | ` ? $
</literallayout>
       For example, <literal>@-</literal> is an allowed operator name,
       but <literal>*-</literal> is not.  This restriction allows
       <productname>PostgreSQL</productname> to parse SQL-compliant
       queries without requiring spaces between tokens.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    When working with non-SQL-standard operator names, you will usually
    need to separate adjacent operators with spaces to avoid ambiguity.
    For example, if you have defined a left unary operator named <literal>@</literal>,
    you cannot write <literal>X*@Y</literal>; you must write
    <literal>X* @Y</literal> to ensure that
    <productname>PostgreSQL</productname> reads it as two operator names
    not one.
   </para>
  </sect2>

  <sect2>
   <title>Special Characters</title>

  <para>
   Some characters that are not alphanumeric have a special meaning
   that is different from being an operator.  Details on the usage can
   be found at the location where the respective syntax element is
   described.  This section only exists to advise the existence and
   summarize the purposes of these characters.

   <itemizedlist>
    <listitem>
     <para>
      A dollar sign (<literal>$</literal>) followed by digits is used
      to represent the positional parameters in the body of a function
      definition.  In other contexts the dollar sign may be part of an
      operator name.
     </para>
    </listitem>

    <listitem>
     <para>
      Parentheses (<literal>()</literal>) have their usual meaning to
      group expressions and enforce precedence.  In some cases
      parentheses are required as part of the fixed syntax of a
      particular SQL command.
     </para>
    </listitem>

    <listitem>
     <para>
      Brackets (<literal>[]</literal>) are used to select the elements
      of an array.  See <xref linkend="arrays"> for more information
      on arrays.
     </para>
    </listitem>

    <listitem>
     <para>
      Commas (<literal>,</literal>) are used in some syntactical
      constructs to separate the elements of a list.
     </para>
    </listitem>

    <listitem>
     <para>
      The semicolon (<literal>;</literal>) terminates an SQL command.
      It cannot appear anywhere within a command, except within a
      string constant or quoted identifier.
     </para>
    </listitem>

    <listitem>
     <para>
      The colon (<literal>:</literal>) is used to select
      <quote>slices</quote> from arrays. (See <xref
      linkend="arrays">.)  In certain SQL dialects (such as Embedded
      SQL), the colon is used to prefix variable names.
     </para>
    </listitem>

    <listitem>
     <para>
      The asterisk (<literal>*</literal>) has a special meaning when
      used in the <command>SELECT</command> command or with the
      <function>COUNT</function> aggregate function.
     </para>
    </listitem>

    <listitem>
     <para>
      The period (<literal>.</literal>) is used in floating-point
      constants, and to separate schema, table, and column names.
     </para>
    </listitem>
   </itemizedlist>

   </para>
  </sect2>

  <sect2 id="sql-syntax-comments">
   <title>Comments</title>

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

   <para>
    A comment is an arbitrary sequence of characters beginning with
    double dashes and extending to the end of the line, e.g.:
<programlisting>
-- This is a standard SQL92 comment
</programlisting>
   </para>

   <para>
    Alternatively, C-style block comments can be used:
<programlisting>
/* multiline comment
 * with nesting: /* nested block comment */
 */
</programlisting>
    where the comment begins with <literal>/*</literal> and extends to
    the matching occurrence of <literal>*/</literal>. These block
    comments nest, as specified in SQL99 but unlike C, so that one can
    comment out larger blocks of code that may contain existing block
    comments.
   </para>

   <para>
    A comment is removed from the input stream before further syntax
    analysis and is effectively replaced by whitespace.
   </para>
  </sect2>

  <sect2 id="sql-precedence">
   <title>Lexical Precedence</title>

   <indexterm zone="sql-precedence">
    <primary>operators</primary>
    <secondary>precedence</secondary>
   </indexterm>

   <para>
    The precedence and associativity of the operators is hard-wired
    into the parser.  Most operators have the same precedence and are
    left-associative.  This may lead to non-intuitive behavior; for
    example the Boolean operators <literal>&lt;</> and <literal>&gt;</> have a different
    precedence than the Boolean operators <literal>&lt;=</> and <literal>&gt;=</>.  Also,
    you will sometimes need to add parentheses when using combinations
    of binary and unary operators.  For instance
<programlisting>
SELECT 5 ! - 6;
</programlisting>
   will be parsed as
<programlisting>
SELECT 5 ! (- 6);
</programlisting>
    because the parser has no idea -- until it is too late -- that
    <token>!</token> is defined as a postfix operator, not an infix one.
    To get the desired behavior in this case, you must write
<programlisting>
SELECT (5 !) - 6;
</programlisting>
    This is the price one pays for extensibility.
   </para>

   <table tocentry="1">
    <title>Operator Precedence (decreasing)</title>

    <tgroup cols="3">
     <thead>
      <row>
       <entry>Operator/Element</entry>
       <entry>Associativity</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry><token>.</token></entry>
       <entry>left</entry>
       <entry>table/column name separator</entry>
      </row>

      <row>
       <entry><token>::</token></entry>
       <entry>left</entry>
       <entry><productname>PostgreSQL</productname>-style typecast</entry>
      </row>

      <row>
       <entry><token>[</token> <token>]</token></entry>
       <entry>left</entry>
       <entry>array element selection</entry>
      </row>

      <row>
       <entry><token>-</token></entry>
       <entry>right</entry>
       <entry>unary minus</entry>
      </row>

      <row>
       <entry><token>^</token></entry>
       <entry>left</entry>
       <entry>exponentiation</entry>
      </row>

      <row>
       <entry><token>*</token> <token>/</token> <token>%</token></entry>
       <entry>left</entry>
       <entry>multiplication, division, modulo</entry>
      </row>

      <row>
       <entry><token>+</token> <token>-</token></entry>
       <entry>left</entry>
       <entry>addition, subtraction</entry>
      </row>

      <row>
       <entry><token>IS</token></entry>
       <entry></entry>
       <entry><literal>IS TRUE</>, <literal>IS FALSE</>, <literal>IS UNKNOWN</>, <literal>IS NULL</></entry>
      </row>

      <row>
       <entry><token>ISNULL</token></entry>
       <entry></entry>
       <entry>test for null</entry>
      </row>

      <row>
       <entry><token>NOTNULL</token></entry>
       <entry></entry>
       <entry>test for not null</entry>
      </row>

      <row>
       <entry>(any other)</entry>
       <entry>left</entry>
       <entry>all other native and user-defined operators</entry>
      </row>

      <row>
       <entry><token>IN</token></entry>
       <entry></entry>
       <entry>set membership</entry>
      </row>

      <row>
       <entry><token>BETWEEN</token></entry>
       <entry></entry>
       <entry>containment</entry>
      </row>

      <row>
       <entry><token>OVERLAPS</token></entry>
       <entry></entry>
       <entry>time interval overlap</entry>
      </row>

      <row>
       <entry><token>LIKE</token> <token>ILIKE</token> <token>SIMILAR</token></entry>
       <entry></entry>
       <entry>string pattern matching</entry>
      </row>

      <row>
       <entry><token>&lt;</token> <token>&gt;</token></entry>
       <entry></entry>
       <entry>less than, greater than</entry>
      </row>

      <row>
       <entry><token>=</token></entry>
       <entry>right</entry>
       <entry>equality, assignment</entry>
      </row>

      <row>
       <entry><token>NOT</token></entry>
       <entry>right</entry>
       <entry>logical negation</entry>
      </row>

      <row>
       <entry><token>AND</token></entry>
       <entry>left</entry>
       <entry>logical conjunction</entry>
      </row>

      <row>
       <entry><token>OR</token></entry>
       <entry>left</entry>
       <entry>logical disjunction</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Note that the operator precedence rules also apply to user-defined
    operators that have the same names as the built-in operators
    mentioned above.  For example, if you define a
    <quote>+</quote> operator for some custom data type it will have
    the same precedence as the built-in <quote>+</quote> operator, no
    matter what yours does.
   </para>

   <para>
    When a schema-qualified operator name is used in the
    <literal>OPERATOR</> syntax, as for example in
<programlisting>
SELECT 3 OPERATOR(pg_catalog.+) 4;
</programlisting>
    the <literal>OPERATOR</> construct is taken to have the default precedence
    shown above for <quote>any other</> operator.  This is true no matter
    which specific operator name appears inside <literal>OPERATOR()</>.
   </para>
  </sect2>
 </sect1>

 <sect1 id="sql-expressions">
  <title>Value Expressions</title>

  <para>
   Value expressions are used in a variety of contexts, such
   as in the target list of the <command>SELECT</command> command, as
   new column values in <command>INSERT</command> or
   <command>UPDATE</command>, or in search conditions in a number of
   commands.  The result of a value expression is sometimes called a
   <firstterm>scalar</firstterm>, to distinguish it from the result of
   a table expression (which is a table).  Value expressions are
   therefore also called <firstterm>scalar expressions</firstterm> (or
   even simply <firstterm>expressions</firstterm>).  The expression
   syntax allows the calculation of values from primitive parts using
   arithmetic, logical, set, and other operations.
  </para>

  <para>
   A value expression is one of the following:

   <itemizedlist>
    <listitem>
     <para>
      A constant or literal value; see <xref linkend="sql-syntax-constants">.
     </para>
    </listitem>

    <listitem>
     <para>
      A column reference.
     </para>
    </listitem>

    <listitem>
     <para>
      A positional parameter reference, in the body of a function declaration.
     </para>
    </listitem>

    <listitem>
     <para>
      An operator invocation.
     </para>
    </listitem>

    <listitem>
     <para>
      A function call.
     </para>
    </listitem>

    <listitem>
     <para>
      An aggregate expression.
     </para>
    </listitem>

    <listitem>
     <para>
      A type cast.
     </para>
    </listitem>

    <listitem>
     <para>
      A scalar subquery.
     </para>
    </listitem>

    <listitem>
<synopsis>( <replaceable>expression</replaceable> )</synopsis>
     <para>
      Parentheses are used to group subexpressions and override precedence.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   In addition to this list, there are a number of constructs that can
   be classified as an expression but do not follow any general syntax
   rules.  These generally have the semantics of a function or
   operator and are explained in the appropriate location in <xref
   linkend="functions">.  An example is the <literal>IS NULL</literal>
   clause.
  </para>

  <para>
   We have already discussed constants in <xref
   linkend="sql-syntax-constants">.  The following sections discuss
   the remaining options.
  </para>

  <sect2>
   <title>Column References</title>

   <para>
    A column can be referenced in the form:
<synopsis>
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable> `['<replaceable>subscript</replaceable>`]'
</synopsis>

    <replaceable>correlation</replaceable> is the name of a
    table (possibly qualified), or an alias for a table defined by means of a
    FROM clause, or 
    the key words <literal>NEW</literal> or <literal>OLD</literal>.
    (NEW and OLD can only appear in rules,
    while other correlation names can be used in any SQL statement.)
    The correlation name and separating dot may be omitted if the column name
    is unique 
    across all the tables being used in the current query.  If
    <replaceable>column</replaceable> is of an array type, then the
    optional <replaceable>subscript</replaceable> selects a specific
    element or elements in the array.  If no subscript is provided, then the
    whole array is selected.  (See <xref linkend="arrays"> for more about
    arrays.)
   </para>
  </sect2>

  <sect2>
   <title>Positional Parameters</title>

   <para>
    A positional parameter reference is used to indicate a parameter
    that is supplied externally to an SQL statement.  Parameters are
    used in SQL function definitions and in prepared queries.
    The form of a parameter reference is:
<synopsis>
$<replaceable>number</replaceable>
</synopsis>
   </para>

   <para>
    For example, consider the definition of a function,
    <function>dept</function>, as

<programlisting>
CREATE FUNCTION dept (text) RETURNS dept
  AS 'SELECT * FROM dept WHERE name = $1'
  LANGUAGE SQL;
</programlisting>

    Here the <literal>$1</literal> will be replaced by the first
    function argument when the function is invoked.
   </para>
  </sect2>

  <sect2>
   <title>Operator Invocations</title>

   <para>
    There are three possible syntaxes for an operator invocation:
    <simplelist>
     <member><replaceable>expression</replaceable> <replaceable>operator</replaceable> <replaceable>expression</replaceable> (binary infix operator)</member>
     <member><replaceable>operator</replaceable> <replaceable>expression</replaceable> (unary prefix operator)</member>
     <member><replaceable>expression</replaceable> <replaceable>operator</replaceable> (unary postfix operator)</member>
    </simplelist>
    where the <replaceable>operator</replaceable> token follows the syntax
    rules of <xref linkend="sql-syntax-operators">, or is one of the
    keywords <token>AND</token>, <token>OR</token>, and
    <token>NOT</token>, or is a qualified operator name
<synopsis>
    <literal>OPERATOR(</><replaceable>schema</><literal>.</><replaceable>operatorname</><literal>)</>
</synopsis>
    Which particular operators exist and whether
    they are unary or binary depends on what operators have been
    defined by the system or the user.  <xref linkend="functions">
    describes the built-in operators.
   </para>
  </sect2>

  <sect2>
   <title>Function Calls</title>

   <para>
    The syntax for a function call is the name of a function
    (possibly qualified with a schema name), followed by its argument list
    enclosed in parentheses:

<synopsis>
<replaceable>function</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional> )
</synopsis>
   </para>

   <para>
    For example, the following computes the square root of 2:
<programlisting>
sqrt(2)
</programlisting>
   </para>

   <para>
    The list of built-in functions is in <xref linkend="functions">.
    Other functions may be added by the user.
   </para>
  </sect2>

  <sect2 id="syntax-aggregates">
   <title>Aggregate Expressions</title>

   <indexterm zone="syntax-aggregates">
    <primary>aggregate functions</primary>
   </indexterm>

   <para>
    An <firstterm>aggregate expression</firstterm> represents the
    application of an aggregate function across the rows selected by a
    query.  An aggregate function reduces multiple inputs to a single
    output value, such as the sum or average of the inputs.  The
    syntax of an aggregate expression is one of the following:

    <simplelist>
     <member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> ( * )</member>
    </simplelist>

    where <replaceable>aggregate_name</replaceable> is a previously
    defined aggregate (possibly a qualified name), and
    <replaceable>expression</replaceable> is 
    any value expression that does not itself contain an aggregate
    expression.
   </para>

   <para>
    The first form of aggregate expression invokes the aggregate
    across all input rows for which the given expression yields a
    non-null value.  (Actually, it is up to the aggregate function
    whether to ignore null values or not --- but all the standard ones do.)
    The second form is the same as the first, since
    <literal>ALL</literal> is the default.  The third form invokes the
    aggregate for all distinct non-null values of the expression found
    in the input rows.  The last form invokes the aggregate once for
    each input row regardless of null or non-null values; since no
    particular input value is specified, it is generally only useful
    for the <function>count()</function> aggregate function.
   </para>

   <para>
    For example, <literal>count(*)</literal> yields the total number
    of input rows; <literal>count(f1)</literal> yields the number of
    input rows in which <literal>f1</literal> is non-null;
    <literal>count(distinct f1)</literal> yields the number of
    distinct non-null values of <literal>f1</literal>.
   </para>

   <para>
    The predefined aggregate functions are described in <xref
    linkend="functions-aggregate">.  Other aggregate functions may be added
    by the user. 
   </para>
  </sect2>

  <sect2 id="sql-syntax-type-casts">
   <title>Type Casts</title>

    <indexterm>
     <primary>data types</primary>
     <secondary>type casts</secondary>
    </indexterm>

   <para>
    A type cast specifies a conversion from one data type to another.
    <productname>PostgreSQL</productname> accepts two equivalent syntaxes
    for type casts:
<synopsis>
CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable> )
<replaceable>expression</replaceable>::<replaceable>type</replaceable>
</synopsis>
    The <literal>CAST</> syntax conforms to SQL92; the syntax with
    <literal>::</literal> is historical <productname>PostgreSQL</productname>
    usage.
   </para>

   <para>
    When a cast is applied to a value expression of a known type, it
    represents a run-time type conversion.  The cast will succeed only
    if a suitable type conversion function is available.  Notice that this
    is subtly different from the use of casts with constants, as shown in
    <xref linkend="sql-syntax-constants-generic">.  A cast applied to an
    unadorned string literal represents the initial assignment of a type
    to a literal constant value, and so it will succeed for any type
    (if the contents of the string literal are acceptable input syntax for the
    data type).
   </para>

   <para>
    An explicit type cast may usually be omitted if there is no ambiguity as
    to the type that a value expression must produce (for example, when it is
    assigned to a table column); the system will automatically apply a
    type cast in such cases.  However, automatic casting is only done for
    cast functions that are marked <quote>OK to apply implicitly</>
    in the system catalogs.  Other cast functions must be invoked with
    explicit casting syntax.  This restriction is intended to prevent
    surprising conversions from being applied silently.
   </para>

   <para>
    It is also possible to specify a type cast using a function-like
    syntax:
<synopsis>
<replaceable>typename</replaceable> ( <replaceable>expression</replaceable> )
</synopsis>
    However, this only works for types whose names are also valid as
    function names.  For example, <literal>double precision</literal>
    can't be used this way, but the equivalent <literal>float8</literal>
    can.  Also, the names <literal>interval</>, <literal>time</>, and
    <literal>timestamp</> can only be used in this fashion if they are
    double-quoted, because of syntactic conflicts.  Therefore, the use of
    the function-like cast syntax leads to inconsistencies and should
    probably be avoided in new applications.
   </para>
  </sect2>

  <sect2>
   <title>Scalar Subqueries</title>

   <para>
    A scalar subquery is an ordinary
    <command>SELECT</command> in parentheses that returns exactly one
    row with one column.  The <command>SELECT</command> query is executed
    and the single returned value is used in the surrounding value expression.
    It is an error to use a query that
    returns more than one row or more than one column as a scalar subquery.
    (But if, during a particular execution, the subquery returns no rows,
    there is no error; the scalar result is taken to be null.)
    The subquery can refer to variables from the surrounding query,
    which will act as constants during any one evaluation of the subquery.
    See also <xref linkend="functions-subquery">.
   </para>

   <para>
    For example, the following finds the largest city population in each
    state:
<programlisting>
SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
FROM states;
</programlisting>
   </para>
  </sect2>

  <sect2>
   <title>Expression Evaluation</title>

   <para>
    The order of evaluation of subexpressions is not defined.  In
    particular, the inputs of an operator or function are not necessarily
    evaluated left-to-right or in any other fixed order.
   </para>

   <para>
    Furthermore, if the result of an expression can be determined by
    evaluating only some parts of it, then other subexpressions
    might not be evaluated at all.  For instance, if one wrote
<programlisting>
SELECT true OR somefunc();
</programlisting>
    then <literal>somefunc()</literal> would (probably) not be called
    at all. The same would be the case if one wrote
<programlisting>
SELECT somefunc() OR true;
</programlisting>
    Note that this is not the same as the left-to-right
    <quote>short-circuiting</quote> of Boolean operators that is found
    in some programming languages.
   </para>

   <para>
    As a consequence, it is unwise to use functions with side effects
    as part of complex expressions.  It is particularly dangerous to
    rely on side effects or evaluation order in WHERE and HAVING clauses,
    since those clauses are extensively reprocessed as part of
    developing an execution plan.  Boolean
    expressions (AND/OR/NOT combinations) in those clauses may be reorganized
    in any manner allowed by the laws of Boolean algebra.
   </para>

   <para>
    When it is essential to force evaluation order, a CASE construct may
    be used.  For example, this is an untrustworthy way of trying to
    avoid division by zero in a WHERE clause:
<programlisting>
SELECT ... WHERE x &lt;&gt; 0 AND y/x &gt; 1.5;
</programlisting>
    but this is safe:
<programlisting>
SELECT ... WHERE CASE WHEN x &lt;&gt; 0 THEN y/x &gt; 1.5 ELSE false END;
</programlisting>
    A CASE construct used in this fashion will defeat optimization attempts,
    so it should only be done when necessary.
   </para>
  </sect2>
 </sect1>

</chapter>

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