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 <chapter id="advanced">
  <title>Advanced <productname>Postgres</productname> <acronym>SQL</acronym> Features</title>

  <para>
   Having covered the basics  of  using
   <productname>Postgres</productname> <acronym>SQL</acronym>  to
   access your data, we will now discuss those features of
   <productname>Postgres</productname> that distinguish  it  from  conventional  data
   managers.   These  features  include  inheritance, time
   travel and non-atomic  data  values  (array-  and  
   set-valued attributes).
   Examples   in   this  section  can  also  be  found  in
   <filename>advance.sql</filename> in the tutorial directory.
   (Refer to <xref linkend="QUERY"> for how to use it.)
  </para>

  <sect1>
   <title>Inheritance</title>

   <para>
    Let's create two classes. The capitals  class  contains
    state  capitals  which  are also cities. Naturally, the
    capitals class should inherit from cities.

    <programlisting>
CREATE TABLE cities (
    name            text,
    population      float,
    altitude        int     -- (in ft)
);

CREATE TABLE capitals (
    state           char(2)
) INHERITS (cities);
    </programlisting>

    In this case, an  instance  of  capitals  <firstterm>inherits</firstterm>  all
    attributes  (name,  population,  and altitude) from its
    parent, cities.  The type  of  the  attribute  name  is
    <type>text</type>,  a  native  <productname>Postgres</productname>
    type  for variable length
    ASCII strings.  The type of the attribute population is
    <type>float</type>,  a  native <productname>Postgres</productname>
    type for double precision
    floating point numbers.  State capitals have  an  extra
    attribute, state, that shows their state.
    In <productname>Postgres</productname>,
    a  class  can inherit from zero or more other classes,
    and a query can reference either  all  instances  of  a
    class  or  all  instances  of  a  class plus all of its
    descendants.

    <note>
     <para>
      The inheritance hierarchy is a  directed  acyclic graph.
     </para>
    </note>

    For example, the  following  query  finds
    all  the cities that are situated at an attitude of 500ft or higher:
     
    <programlisting>
SELECT name, altitude
    FROM cities
    WHERE altitude &gt; 500;

+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
    </programlisting>         
   </para>

   <para>
    On the other hand, to find the  names  of  all  cities,
    including  state capitals, that are located at an altitude 
    over 500ft, the query is:

    <programlisting>
SELECT c.name, c.altitude
    FROM cities* c
    WHERE c.altitude > 500;
    </programlisting>

    which returns:
     
    <programlisting>
+----------+----------+
|name      | altitude |
+----------+----------+
|Las Vegas | 2174     |
+----------+----------+
|Mariposa  | 1953     |
+----------+----------+
|Madison   | 845      |
+----------+----------+
    </programlisting>

    Here the <quote>*</quote> after cities indicates that the query should
    be  run over cities and all classes below cities in the
    inheritance hierarchy.  Many of the  commands  that  we
    have  already discussed (<command>SELECT</command>,
    <command>UPDATE</command> and <command>DELETE</command>)
    support this <quote>*</quote> notation, as do others, like
    <command>ALTER</command>.
   </para>
  </sect1>

  <sect1>
   <title>Non-Atomic Values</title>

   <para>
    One  of  the tenets of the relational model is that the
    attributes of a relation are atomic.
    <productname>Postgres</productname> does not
    have  this  restriction; attributes can themselves contain 
    sub-values that can be  accessed  from  the  query
    language.   For example, you can create attributes that
    are arrays of base types.
   </para>

   <sect2>
    <title>Arrays</title>

    <para>
     <productname>Postgres</productname> allows attributes of an
     instance to be defined
     as  fixed-length  or  variable-length multi-dimensional
     arrays. Arrays of any base type  or  user-defined  type
     can  be created. To illustrate their use, we first create a 
     class with arrays of base types.

     <programlisting>
CREATE TABLE SAL_EMP (
    name            text,
    pay_by_quarter  int4[],
    schedule        text[][]
);
     </programlisting>
    </para>

    <para>
     The above query will create a class named SAL_EMP  with
     a  <firstterm>text</firstterm>  string (name), a one-dimensional
     array of <firstterm>int4</firstterm>
     (pay_by_quarter),  which  represents   the   employee's
     salary by quarter and a two-dimensional array of
     <firstterm>text</firstterm>
     (schedule),  which  represents  the  employee's  weekly
     schedule.   Now  we  do  some  <firstterm>INSERTS</firstterm>s;
     note that when
     appending to an array, we  enclose  the  values  within
     braces  and  separate  them  by commas.  If you know
     <firstterm>C</firstterm>,
     this is not unlike the syntax for  initializing  structures.

     <programlisting>
INSERT INTO SAL_EMP
    VALUES ('Bill',
    '{10000, 10000, 10000, 10000}',
    '{{"meeting", "lunch"}, {}}');

INSERT INTO SAL_EMP
    VALUES ('Carol',
    '{20000, 25000, 25000, 25000}',
    '{{"talk", "consult"}, {"meeting"}}');
     </programlisting>

     By  default,  <productname>Postgres</productname>  uses  the
     "one-based" numbering
     convention for arrays -- that is, an array  of  n  elements
     starts with array[1] and ends with array[n].
     Now,  we  can  run  some queries on SAL_EMP.  First, we
     show how to access a single element of an  array  at  a
     time.   This query retrieves the names of the employees
     whose pay changed in the second quarter:

     <programlisting>
SELECT name
    FROM SAL_EMP
    WHERE SAL_EMP.pay_by_quarter[1] &lt;&gt;
    SAL_EMP.pay_by_quarter[2];

+------+
|name  |
+------+
|Carol |
+------+
     </programlisting>
    </para>

    <para>
     This query retrieves  the  third  quarter  pay  of  all
     employees:
     
     <programlisting>
SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;


+---------------+
|pay_by_quarter |
+---------------+
|10000          |
+---------------+
|25000          |
+---------------+
     </programlisting>
    </para>

    <para>
     We  can  also  access  arbitrary slices of an array, or
     subarrays.  This query  retrieves  the  first  item  on
     Bill's schedule for the first two days of the week.

     <programlisting>
SELECT SAL_EMP.schedule[1:2][1:1]
    FROM SAL_EMP
    WHERE SAL_EMP.name = 'Bill';

+-------------------+
|schedule           |
+-------------------+
|{{"meeting"},{""}} |
+-------------------+
     </programlisting>
    </para>
   </sect2>
  </sect1>

  <sect1>
   <title>More Advanced Features</title>

   <para>
    <productname>Postgres</productname> has many features not touched
    upon in this
    tutorial introduction, which has been oriented toward newer users of
    <acronym>SQL</acronym>.
    These are discussed in more detail in both the User's and
    Programmer's Guides.
   </para>

  </sect1>

 </chapter>

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