Add User's Guide chapters on Data Definition and Data Manipulation.

[pg-rex/syncrep.git] / doc / src / sgml / 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 the
   <citetitle>Reference Manual</citetitle>.
  </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 32 so the maximum identifier length
    is 31 (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 four kinds of <firstterm>implicitly-typed
    constants</firstterm> in <productname>PostgreSQL</productname>:
    strings, bit strings, integers, and floating-point 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, and <productname>PostgreSQL</productname> is
      consistent with <acronym>SQL9x</acronym> in this regard.
    </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>.  Bit-string constants can be continued
     across lines in the same way as regular string constants.
    </para>
   </sect3>

   <sect3>
    <title>Integer Constants</title>

    <para>
     Integer constants in SQL are sequences of decimal digits (0
     though 9) with no decimal point and no exponent.  The range of legal values
     depends on which integer data type is used, but the plain
     <type>integer</type> type accepts values ranging from -2147483648
     to +2147483647.  (The optional plus or minus sign is actually a
     separate unary operator and not part of the integer constant.)
    </para>
   </sect3>

   <sect3>
    <title>Floating-Point Constants</title>

    <indexterm>
     <primary>floating point</primary>
     <secondary>constants</secondary>
    </indexterm>

    <para>
     Floating-point constants are accepted in these general forms:
<synopsis>
<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.  At least one digit must be before or after the decimal
     point. At least one digit must follow the exponent delimiter
      (<literal>e</literal>) if that field is present.
     Thus, a floating-point constant is distinguished from an integer
     constant by the presence of either the decimal point or the
     exponent clause (or both).  There must not be a space or other
     characters embedded in the constant.
    </para>

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

    <para>
     Floating-point constants are of type <type>DOUBLE
     PRECISION</type>. <type>REAL</type> can be specified explicitly
     by using <acronym>SQL</acronym> string notation or
     <productname>PostgreSQL</productname> type notation:

<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
    (31 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 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>test for TRUE, FALSE, UNKNOWN, 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></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>
  </sect2>
 </sect1>

 <sect1 id="sql-naming">
  <title>Schemas and Naming Conventions</title>

     <indexterm>
      <primary>schemas</primary>
     </indexterm>

     <indexterm>
      <primary>search path</primary>
     </indexterm>

     <indexterm>
      <primary>namespaces</primary>
     </indexterm>

   <para>
    A <productname>PostgreSQL</productname> database cluster (installation)
    contains one or more named databases.  Users and groups of users are
    shared across the entire cluster, but no other data is shared across
    databases.  Any given client connection to the server can access
    only the data in a single database, the one specified in the connection
    request.
   </para>

   <note>
    <para>
     Users of a cluster do not necessarily have the privilege to access every
     database in the cluster.  Sharing of user names means that there
     cannot be different users named, say, <literal>joe</> in two databases
     in the same cluster; but the system can be configured to allow
     <literal>joe</> access to only some of the databases.
    </para>
   </note>

  <sect2>
   <title>Schema Object Names</title>

   <para>
    A database contains one or more named <firstterm>schemas</>, which
    in turn contain tables.  Schemas also contain other kinds of named
    objects, including datatypes, functions, and operators.  The same
    object name can be used in different schemas without conflict; for
    example, both <literal>schema1</> and <literal>myschema</> may
    contain tables named <literal>mytable</>.  Unlike databases, schemas
    are not rigidly separated: a user may access objects in any of the
    schemas in the database he is connected to, if he has privileges
    to do so.
   </para>

     <indexterm>
      <primary>qualified names</primary>
     </indexterm>

     <indexterm>
      <primary>names</primary>
      <secondary>qualified</secondary>
     </indexterm>

   <para>
    To name a table precisely, write a <firstterm>qualified name</> consisting
    of the schema name and table name separated by a dot:
<synopsis>
    <replaceable>schema</><literal>.</><replaceable>table</>
</synopsis>
    Actually, the even more general syntax
<synopsis>
    <replaceable>database</><literal>.</><replaceable>schema</><literal>.</><replaceable>table</>
</synopsis>
    can be used too, but at present this is just for pro-forma compliance
    with the SQL standard; if you write a database name it must be the
    same as the database you are connected to.
   </para>

     <indexterm>
      <primary>unqualified names</primary>
     </indexterm>

     <indexterm>
      <primary>names</primary>
      <secondary>unqualified</secondary>
     </indexterm>

   <para>
    Qualified names are tedious to write, and it's often best not to
    wire a particular schema name into applications anyway.  Therefore
    tables are often referred to by <firstterm>unqualified names</>,
    which consist of just the table name.  The system determines which table
    is meant by following a <firstterm>search path</>, which is a list
    of schemas to look in.  The first matching table in the search path
    is taken to be the one wanted.  If there is no match in the search
    path, an error is reported, even if matching table names exist
    in other schemas in the database.
   </para>

   <para>
    The first schema named in the search path is called the current schema.
    Aside from being the first schema searched, it is also the schema in
    which new tables will be created if the <command>CREATE TABLE</>
    command does not specify a schema name.
   </para>

   <para>
    The search path works in the same way for datatype names, function names,
    and operator names as it does for table names.  Datatype and function
    names can be qualified in exactly the same way as table names.  If you
    need to write a qualified operator name in an expression, there is a
    special provision: you must write
<synopsis>
<literal>OPERATOR(</><replaceable>schema</><literal>.</><replaceable>operator</><literal>)</>
</synopsis>
    This is needed to avoid syntactic ambiguity.  An example is
<programlisting>
SELECT 3 OPERATOR(pg_catalog.+) 4;
</programlisting>
    In practice one usually relies on the search path for operators,
    so as not to have to write anything so ugly as that.
   </para>

   <para>
    The standard search path in <productname>PostgreSQL</productname>
    contains first the schema having the same name as the session user
    (if it exists), and second the schema named <literal>public</>
    (if it exists, which it does by default).  This arrangement allows
    a flexible combination of private and shared tables.  If no per-user
    schemas are created then all user tables will exist in the shared
    <literal>public</> schema, providing behavior that is backwards-compatible
    with pre-7.3 <productname>PostgreSQL</productname> releases.
   </para>

   <note>
    <para>
     There is no concept of a <literal>public</> schema in the SQL standard.
     To achieve closest conformance to the standard, the DBA should
     create per-user schemas for every user, and not use (perhaps even
     remove) the <literal>public</> schema.
    </para>
   </note>

   <para>
    In addition to <literal>public</> and user-created schemas, each database
    contains a 
    <literal>pg_catalog</> schema, which contains the system tables
    and all the built-in datatypes, functions, and operators.
    <literal>pg_catalog</> is always effectively part of the search path.
    If it is not named explicitly in the path then it is implicitly searched
    <emphasis>before</> searching the path's schemas.  This ensures that
    built-in names will always be findable.  However, you may explicitly
    place <literal>pg_catalog</> at the end of your search path if you
    prefer to have user-defined names override built-in names.
   </para>
  </sect2>

  <sect2 id="sql-reserved-names">
   <title>Reserved names</title>

     <indexterm>
      <primary>reserved names</primary>
     </indexterm>

     <indexterm>
      <primary>names</primary>
      <secondary>reserved</secondary>
     </indexterm>

    <para>
     There are several restrictions on the names that can be chosen for
     user-defined database objects.  These restrictions vary depending
     on the kind of object.  (Note that these restrictions are
     separate from whether the name is a key word or not; quoting a
     name will not allow you to escape these restrictions.)
    </para>

    <para>
     Schema names beginning with <literal>pg_</> are reserved for system
     purposes and may not be created by users.
    </para>

    <para>
     In <productname>PostgreSQL</productname> versions before 7.3, table
     names beginning with <literal>pg_</> were reserved.  This is no longer
     true: you may create such a table name if you wish, in any non-system
     schema.  However, it's best to continue to avoid such names,
     to ensure that you won't suffer a conflict if some future version
     defines a system catalog named the same as your table.  (With the
     default search path, an unqualified reference to your table name
     would be resolved as the system catalog instead.)  System catalogs will
     continue to follow the convention of having names beginning with
     <literal>pg_</>, so that they will not conflict with unqualified
     user-table names so long as users avoid the <literal>pg_</> prefix.
    </para>

    <para>
     Every table has several <firstterm>system columns</> that are
     implicitly defined by the system.  Therefore, these names cannot
     be used as names of user-defined columns:

     <indexterm>
      <primary>columns</primary>
      <secondary>system columns</secondary>
     </indexterm>

     <variablelist>
      <varlistentry>
       <term><structfield>oid</></term>
       <listitem>
        <para>
         <indexterm>
          <primary>OID</primary>
         </indexterm>
         The object identifier (object ID) of a row.  This is a serial number
         that is automatically added by <productname>PostgreSQL</productname> to all table rows (unless
         the table was created WITHOUT OIDS, in which case this column is
         not present).  See <xref linkend="datatype-oid"> for more info.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term><structfield>tableoid</></term>
       <listitem>
        <para>
         The OID of the table containing this row.  This attribute is
         particularly handy for queries that select from inheritance
         hierarchies, since without it, it's difficult to tell which
         individual table a row came from.  The
         <structfield>tableoid</structfield> can be joined against the
         <structfield>oid</structfield> column of
         <classname>pg_class</classname> to obtain the table name.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><structfield>xmin</></term>
       <listitem>
        <para>
         The identity (transaction ID) of the inserting transaction for
         this tuple.  (Note: A tuple is an individual state of a row;
         each update of a row creates a new tuple for the same logical row.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term><structfield>cmin</></term>
       <listitem>
        <para>
         The command identifier (starting at zero) within the inserting
         transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term><structfield>xmax</></term>
       <listitem>
        <para>
         The identity (transaction ID) of the deleting transaction,
         or zero for an undeleted tuple.  It is possible for this field
         to be nonzero in a visible tuple: that usually indicates that the
         deleting transaction hasn't committed yet, or that an attempted
         deletion was rolled back.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term><structfield>cmax</></term>
       <listitem>
        <para>
         The command identifier within the deleting transaction, or zero.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term><structfield>ctid</></term>
       <listitem>
        <para>
         The physical location of the tuple within its table.
         Note that although the <structfield>ctid</structfield>
         can be used to locate the tuple very quickly, a row's
         <structfield>ctid</structfield> will change each time it is updated
         or moved by <command>VACUUM FULL</>.
         Therefore <structfield>ctid</structfield> is useless as a long-term
         row identifier.
         The OID, or even better a user-defined serial number, should
         be used to identify logical rows.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </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 the action portion of a rule,
    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
    in an SQL function.  Typically this is used in SQL function
    definition statements.  The form of a parameter 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 NULLs 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>okay 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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