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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>

  <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>
   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 <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 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>$</>).  Note that dollar signs are not allowed in identifiers
    according to the letter of the SQL standard, so their use may 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
    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. If this limit is problematic, it can be raised by
    changing the <symbol>NAMEDATALEN</symbol> constant in
    <filename>src/include/postgres_ext.h</filename>.
   </para>

   <para>
    <indexterm>
     <primary>case sensitivity</primary>
     <secondary>of 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>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>
   </para>

   <para>
    Quoted identifiers can contain any character other than a double
    quote itself.  (To include a double quote, write two double quotes.)
    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.  (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>
  </sect2>


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

   <indexterm zone="sql-syntax-constants">
    <primary>constant</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. These alternatives are discussed in the following
    subsections.
   </para>

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

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

    <para>
     <indexterm>
      <primary>quotation marks</primary>
      <secondary>escaping</secondary>
     </indexterm>
     A string constant in SQL is an arbitrary sequence of characters
     bounded by single quotes (<literal>'</literal>), for example
     <literal>'This is a string'</literal>.  The standard-compliant way of
     writing a single-quote character within a string constant is to
     write two adjacent single quotes, e.g.
     <literal>'Dianne''s horse'</literal>.
     <productname>PostgreSQL</productname> also allows single quotes
     to be escaped with a backslash (<literal>\'</literal>).  However,
     future versions of <productname>PostgreSQL</productname> will not
     allow this, so applications using backslashes should convert to the 
     standard-compliant method outlined above.
    </para>

    <para>
     Another <productname>PostgreSQL</productname> extension is that
     C-style backslash escapes are 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. Also supported is
     <literal>\<replaceable>digits</replaceable></literal>, where
     <replaceable>digits</replaceable> represents an octal byte value, and
     <literal>\x<replaceable>hexdigits</replaceable></literal>, where
     <replaceable>hexdigits</replaceable> represents a hexadecimal byte value.
     (It is your responsibility that the byte sequences you create are
     valid characters in the server character set encoding.) Any other
     character following a backslash is taken literally. Thus, to
     include a backslash in a string constant, write two backslashes.
    </para>

    <note>
    <para>
     While ordinary strings now support C-style backslash escapes,
     future versions will generate warnings for such usage and
     eventually treat backslashes as literal characters to be
     standard-conforming. The proper way to specify escape processing is
     to use the escape string syntax to indicate that escape
     processing is desired. Escape string syntax is specified by writing
     the letter <literal>E</literal> (upper or lower case) just before
     the string, e.g. <literal>E'\041'</>. This method will work in all
     future versions of <productname>PostgreSQL</productname>.
    </para>
    </note>

    <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>SQL</acronym>; <productname>PostgreSQL</productname> is
     following the standard.)
    </para>
   </sect3>

   <sect3 id="sql-syntax-dollar-quoting">
    <title>Dollar-Quoted String Constants</title>

     <indexterm>
      <primary>dollar quoting</primary>
     </indexterm>

    <para>
     While the standard syntax for specifying string constants is usually
     convenient, it can be difficult to understand when the desired string
     contains many single quotes or backslashes, since each of those must
     be doubled. To allow more readable queries in such situations,
     <productname>PostgreSQL</productname> provides another way, called
     <quote>dollar quoting</quote>, to write string constants.
     A dollar-quoted string constant
     consists of a dollar sign (<literal>$</literal>), an optional
     <quote>tag</quote> of zero or more characters, another dollar
     sign, an arbitrary sequence of characters that makes up the
     string content, a dollar sign, the same tag that began this
     dollar quote, and a dollar sign. For example, here are two
     different ways to specify the string <quote>Dianne's horse</>
     using dollar quoting:
<programlisting>
$$Dianne's horse$$
$SomeTag$Dianne's horse$SomeTag$
</programlisting>
     Notice that inside the dollar-quoted string, single quotes can be
     used without needing to be escaped.  Indeed, no characters inside
     a dollar-quoted string are ever escaped: the string content is always
     written literally.  Backslashes are not special, and neither are
     dollar signs, unless they are part of a sequence matching the opening
     tag.
    </para>

    <para>
     It is possible to nest dollar-quoted string constants by choosing
     different tags at each nesting level.  This is most commonly used in
     writing function definitions.  For example:
<programlisting>
$function$
BEGIN
    RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
END;
$function$
</programlisting>
     Here, the sequence <literal>$q$[\t\r\n\v\\]$q$</> represents a
     dollar-quoted literal string <literal>[\t\r\n\v\\]</>, which will
     be recognized when the function body is executed by
     <productname>PostgreSQL</>.  But since the sequence does not match
     the outer dollar quoting delimiter <literal>$function$</>, it is
     just some more characters within the constant so far as the outer
     string is concerned.
    </para>

    <para>
     The tag, if any, of a dollar-quoted string follows the same rules
     as an unquoted identifier, except that it cannot contain a dollar sign.
     Tags are case sensitive, so <literal>$tag$String content$tag$</literal>
     is correct, but <literal>$TAG$String content$tag$</literal> is not.
    </para>

    <para>
     A dollar-quoted string that follows a keyword or identifier must
     be separated from it by whitespace; otherwise the dollar quoting
     delimiter would be taken as part of the preceding identifier.
    </para>

    <para>
     Dollar quoting is not part of the SQL standard, but it is often a more
     convenient way to write complicated string literals than the
     standard-compliant single quote syntax.  It is particularly useful when
     representing string constants inside other constants, as is often needed
     in procedural function definitions.  With single-quote syntax, each
     backslash in the above example would have to be written as four
     backslashes, which would be reduced to two backslashes in parsing the
     original string constant, and then to one when the inner string constant
     is re-parsed during function execution.
    </para>
   </sect3>

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

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

    <para>
     Bit-string constants look like regular 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 hexadecimal digit.
    </para>

    <para>
     Both forms of bit-string constant can be continued
     across lines in the same way as regular string constants.
     Dollar quoting cannot be used in a bit-string constant.
    </para>
   </sect3>

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

    <indexterm>
     <primary>number</primary>
     <secondary>constant</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.  Note 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>
     <indexterm><primary>integer</primary></indexterm>
     <indexterm><primary>bigint</primary></indexterm>
     <indexterm><primary>numeric</primary></indexterm>
     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.<indexterm><primary>type cast</primary></indexterm>
     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>

     These are actually just special cases of the general casting
     notations discussed next.
    </para>
   </sect3>

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

    <indexterm>
     <primary>data type</primary>
     <secondary>constant</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 constant'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 assigned directly to a
     table column), in which case it is automatically coerced.
    </para>

    <para>
     The string constant can be written using either regular SQL
     notation or dollar-quoting.
    </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
     <literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
     can only be used to specify the type of a literal constant.
     Another restriction on
     <literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
     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>

    <para>
     The <literal>CAST()</> syntax conforms to SQL.  The
     <literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
     syntax is a generalization of the standard: SQL specifies this syntax only
     for a few data types, but <productname>PostgreSQL</productname> allows it
     for all types.  The syntax with
     <literal>::</literal> is historical <productname>PostgreSQL</productname>
     usage, as is the function-call syntax.
    </para>
   </sect3>
  </sect2>

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

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

   <para>
    An operator name 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>--</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 a positional parameter in the body of a function
      definition or a prepared statement.  In other contexts the
      dollar sign may be part of an identifier or a dollar-quoted string
      constant.
     </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>) is used in some contexts to denote
      all the fields of a table row or composite value.  It also
      has a special meaning when used as the argument of the
      <function>COUNT</function> aggregate function.
     </para>
    </listitem>

    <listitem>
     <para>
      The period (<literal>.</literal>) is used in numeric
      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>comment</primary>
    <secondary sortas="SQL">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 SQL 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 the SQL standard 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>operator</primary>
    <secondary>precedence</secondary>
   </indexterm>

   <para>
    <xref linkend="sql-precedence-table"> shows the precedence and
    associativity of the operators in <productname>PostgreSQL</>.
    Most operators have the same precedence and are left-associative.
    The precedence and associativity of the operators is hard-wired
    into the parser.  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 &mdash; until it is too late
    &mdash; 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 id="sql-precedence-table">
    <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>range 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 in <xref linkend="sql-precedence-table"> 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>

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

  <indexterm zone="sql-expressions">
   <primary>value expression</primary>
  </indexterm>

  <indexterm>
   <primary>scalar</primary>
   <see>expression</see>
  </indexterm>

  <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.
     </para>
    </listitem>

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

    <listitem>
     <para>
      A positional parameter reference, in the body of a function definition
      or prepared statement.
     </para>
    </listitem>

    <listitem>
     <para>
      A subscripted expression.
     </para>
    </listitem>

    <listitem>
     <para>
      A field selection expression.
     </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>
     <para>
      An array constructor.
     </para>
    </listitem>

    <listitem>
     <para>
      A row constructor.
     </para>
    </listitem>

    <listitem>
     <para>
      Another value expression in parentheses, useful to group
      subexpressions and override
      precedence.<indexterm><primary>parenthesis</></>
     </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>

   <indexterm>
    <primary>column reference</primary>
   </indexterm>

   <para>
    A column can be referenced in the form
<synopsis>
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable>
</synopsis>
   </para>

   <para>
    <replaceable>correlation</replaceable> is the name of a
    table (possibly qualified with a schema name), or an alias for a table
    defined by means of a <literal>FROM</literal> clause, or one of
    the key words <literal>NEW</literal> or <literal>OLD</literal>.
    (<literal>NEW</literal> and <literal>OLD</literal> can only appear in rewrite 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.  (See also <xref linkend="queries">.)
   </para>
  </sect2>

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

   <indexterm>
    <primary>parameter</primary>
    <secondary>syntax</secondary>
   </indexterm>

   <indexterm>
    <primary>$</primary>
   </indexterm>

   <para>
    A positional parameter reference is used to indicate a value
    that is supplied externally to an SQL statement.  Parameters are
    used in SQL function definitions and in prepared queries.  Some
    client libraries also support specifying data values separately
    from the SQL command string, in which case parameters are used to
    refer to the out-of-line data values.
    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> references the value of the first
    function argument whenever the function is invoked.
   </para>
  </sect2>

  <sect2>
   <title>Subscripts</title>

   <indexterm>
    <primary>subscript</primary>
   </indexterm>

   <para>
    If an expression yields a value of an array type, then a specific
    element of the array value can be extracted by writing
<synopsis>
<replaceable>expression</replaceable>[<replaceable>subscript</replaceable>]
</synopsis>
    or multiple adjacent elements (an <quote>array slice</>) can be extracted
    by writing
<synopsis>
<replaceable>expression</replaceable>[<replaceable>lower_subscript</replaceable>:<replaceable>upper_subscript</replaceable>]
</synopsis>
    (Here, the brackets <literal>[ ]</literal> are meant to appear literally.)
    Each <replaceable>subscript</replaceable> is itself an expression,
    which must yield an integer value.
   </para>

   <para>
    In general the array <replaceable>expression</replaceable> must be
    parenthesized, but the parentheses may be omitted when the expression
    to be subscripted is just a column reference or positional parameter.
    Also, multiple subscripts can be concatenated when the original array
    is multidimensional.
    For example,

<programlisting>
mytable.arraycolumn[4]
mytable.two_d_column[17][34]
$1[10:42]
(arrayfunction(a,b))[42]
</programlisting>

    The parentheses in the last example are required.
    See <xref linkend="arrays"> for more about arrays.
   </para>
  </sect2>

  <sect2>
   <title>Field Selection</title>

   <indexterm>
    <primary>field selection</primary>
   </indexterm>

   <para>
    If an expression yields a value of a composite type (row type), then a
    specific field of the row can be extracted by writing
<synopsis>
<replaceable>expression</replaceable>.<replaceable>fieldname</replaceable>
</synopsis>
   </para>

   <para>
    In general the row <replaceable>expression</replaceable> must be
    parenthesized, but the parentheses may be omitted when the expression
    to be selected from is just a table reference or positional parameter.
    For example,

<programlisting>
mytable.mycolumn
$1.somecolumn
(rowfunction(a,b)).col3
</programlisting>

    (Thus, a qualified column reference is actually just a special case
    of the field selection syntax.)
   </para>
  </sect2>

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

   <indexterm>
    <primary>operator</primary>
    <secondary>invocation</secondary>
   </indexterm>

   <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
    key words <token>AND</token>, <token>OR</token>, and
    <token>NOT</token>, or is a qualified operator name in the form
<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>

   <indexterm>
    <primary>function</primary>
    <secondary>invocation</secondary>
   </indexterm>

   <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 function</primary>
    <secondary>invocation</secondary>
   </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:

<synopsis>
<replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)
<replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)
<replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)
<replaceable>aggregate_name</replaceable> ( * )
</synopsis>

    where <replaceable>aggregate_name</replaceable> is a previously
    defined aggregate (possibly qualified with a schema 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 &mdash; 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>

   <para>
    An aggregate expression may only appear in the result list or
    <literal>HAVING</> clause of a <command>SELECT</> command.
    It is forbidden in other clauses, such as <literal>WHERE</>,
    because those clauses are logically evaluated before the results
    of aggregates are formed.
   </para>

   <para>
    When an aggregate expression appears in a subquery (see
    <xref linkend="sql-syntax-scalar-subqueries"> and
    <xref linkend="functions-subquery">), the aggregate is normally
    evaluated over the rows of the subquery.  But an exception occurs
    if the aggregate's argument contains only outer-level variables:
    the aggregate then belongs to the nearest such outer level, and is
    evaluated over the rows of that query.  The aggregate expression
    as a whole is then an outer reference for the subquery it appears in,
    and acts as a constant over any one evaluation of that subquery.
    The restriction about
    appearing only in the result list or <literal>HAVING</> clause
    applies with respect to the query level that the aggregate belongs to.
   </para>
  </sect2>

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

   <indexterm>
    <primary>data type</primary>
    <secondary>type cast</secondary>
   </indexterm>

   <indexterm>
    <primary>type cast</primary>
   </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 SQL; 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 operation has been defined.  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
    casts that are marked <quote>OK to apply implicitly</>
    in the system catalogs.  Other casts 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.

    (The function-like syntax is in fact just a function call.  When
    one of the two standard cast syntaxes is used to do a run-time
    conversion, it will internally invoke a registered function to
    perform the conversion.  By convention, these conversion functions
    have the same name as their output type, and thus the <quote>function-like
    syntax</> is nothing more than a direct invocation of the underlying
    conversion function.  Obviously, this is not something that a portable
    application should rely on.)
   </para>
  </sect2>

  <sect2 id="sql-syntax-scalar-subqueries">
   <title>Scalar Subqueries</title>

   <indexterm>
    <primary>subquery</primary>
   </indexterm>

   <para>
    A scalar subquery is an ordinary
    <command>SELECT</command> query in parentheses that returns exactly one
    row with one column.  (See <xref linkend="queries"> for information about writing queries.)
    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"> for other expressions involving subqueries.
   </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 id="sql-syntax-array-constructors">
   <title>Array Constructors</title>

   <indexterm>
    <primary>array</primary>
    <secondary>constructor</secondary>
   </indexterm>

   <indexterm>
    <primary>ARRAY</primary>
   </indexterm>

   <para>
    An array constructor is an expression that builds an
    array value from values for its member elements.  A simple array
    constructor 
    consists of the key word <literal>ARRAY</literal>, a left square bracket
    <literal>[</>, one or more expressions (separated by commas) for the
    array element values, and finally a right square bracket <literal>]</>.
    For example,
<programlisting>
SELECT ARRAY[1,2,3+4];
  array
---------
 {1,2,7}
(1 row)
</programlisting>
    The array element type is the common type of the member expressions,
    determined using the same rules as for <literal>UNION</> or
    <literal>CASE</> constructs (see <xref linkend="typeconv-union-case">). 
   </para>

   <para>
    Multidimensional array values can be built by nesting array
    constructors.
    In the inner constructors, the key word <literal>ARRAY</literal> may
    be omitted.  For example, these produce the same result:

<programlisting>
SELECT ARRAY[ARRAY[1,2], ARRAY[3,4]];
     array
---------------
 {{1,2},{3,4}}
(1 row)

SELECT ARRAY[[1,2],[3,4]];
     array
---------------
 {{1,2},{3,4}}
(1 row)
</programlisting>

    Since multidimensional arrays must be rectangular, inner constructors
    at the same level must produce sub-arrays of identical dimensions.
  </para>

  <para>
    Multidimensional array constructor elements can be anything yielding
    an array of the proper kind, not only a sub-<literal>ARRAY</> construct.
    For example:
<programlisting>
CREATE TABLE arr(f1 int[], f2 int[]);

INSERT INTO arr VALUES (ARRAY[[1,2],[3,4]], ARRAY[[5,6],[7,8]]);

SELECT ARRAY[f1, f2, '{{9,10},{11,12}}'::int[]] FROM arr;
                     array
------------------------------------------------
 {{{1,2},{3,4}},{{5,6},{7,8}},{{9,10},{11,12}}}
(1 row)
</programlisting>
  </para>

  <para>
   It is also possible to construct an array from the results of a
   subquery.  In this form, the array constructor is written with the
   key word <literal>ARRAY</literal> followed by a parenthesized (not
   bracketed) subquery. For example:
<programlisting>
SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
                          ?column?
-------------------------------------------------------------
 {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
(1 row)
</programlisting>
   The subquery must return a single column. The resulting
   one-dimensional array will have an element for each row in the
   subquery result, with an element type matching that of the
   subquery's output column.
  </para>

  <para>
   The subscripts of an array value built with <literal>ARRAY</literal>
   always begin with one.  For more information about arrays, see
   <xref linkend="arrays">.
  </para>

  </sect2>

  <sect2 id="sql-syntax-row-constructors">
   <title>Row Constructors</title>

   <indexterm>
    <primary>composite type</primary>
    <secondary>constructor</secondary>
   </indexterm>

   <indexterm>
    <primary>row type</primary>
    <secondary>constructor</secondary>
   </indexterm>

   <indexterm>
    <primary>ROW</primary>
   </indexterm>

   <para>
    A row constructor is an expression that builds a row value (also
    called a composite value) from values
    for its member fields.  A row constructor consists of the key word
    <literal>ROW</literal>, a left parenthesis, zero or more
    expressions (separated by commas) for the row field values, and finally
    a right parenthesis.  For example,
<programlisting>
SELECT ROW(1,2.5,'this is a test');
</programlisting>
    The key word <literal>ROW</> is optional when there is more than one
    expression in the list.
   </para>

   <para>
    By default, the value created by a <literal>ROW</> expression is of
    an anonymous record type.  If necessary, it can be cast to a named
    composite type &mdash; either the row type of a table, or a composite type
    created with <command>CREATE TYPE AS</>.  An explicit cast may be needed
    to avoid ambiguity.  For example:
<programlisting>
CREATE TABLE mytable(f1 int, f2 float, f3 text);

CREATE FUNCTION getf1(mytable) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

-- No cast needed since only one getf1() exists
SELECT getf1(ROW(1,2.5,'this is a test'));
 getf1
-------
     1
(1 row)

CREATE TYPE myrowtype AS (f1 int, f2 text, f3 numeric);

CREATE FUNCTION getf1(myrowtype) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

-- Now we need a cast to indicate which function to call:
SELECT getf1(ROW(1,2.5,'this is a test'));
ERROR:  function getf1(record) is not unique

SELECT getf1(ROW(1,2.5,'this is a test')::mytable);
 getf1
-------
     1
(1 row)

SELECT getf1(CAST(ROW(11,'this is a test',2.5) AS myrowtype));
 getf1
-------
    11
(1 row)
</programlisting>
  </para>

  <para>
   Row constructors can be used to build composite values to be stored
   in a composite-type table column, or to be passed to a function that
   accepts a composite parameter.  Also,
   it is possible to compare two row values or test a row with
   <literal>IS NULL</> or <literal>IS NOT NULL</>, for example
<programlisting>
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');

SELECT ROW(a, b, c) IS NOT NULL FROM table;
</programlisting>
   For more detail see <xref linkend="functions-comparisons">.
   Row constructors can also be used in connection with subqueries,
   as discussed in <xref linkend="functions-subquery">.
  </para>

  </sect2>

  <sect2 id="syntax-express-eval">
   <title>Expression Evaluation Rules</title>

   <indexterm>
    <primary>expression</primary>
    <secondary>order of evaluation</secondary>
   </indexterm>

   <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 <literal>WHERE</> and <literal>HAVING</> clauses,
    since those clauses are extensively reprocessed as part of
    developing an execution plan.  Boolean
    expressions (<literal>AND</>/<literal>OR</>/<literal>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 <literal>CASE</>
    construct (see <xref linkend="functions-conditional">) may be
    used.  For example, this is an untrustworthy way of trying to
    avoid division by zero in a <literal>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 <literal>CASE</> construct used in this fashion will defeat optimization
    attempts, so it should only be done when necessary.  (In this particular
    example, it would doubtless be best to sidestep the problem by writing
    <literal>y &gt; 1.5*x</> instead.)
   </para>
  </sect2>
 </sect1>

</chapter>

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