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$Header: /cvsroot/pgsql/doc/src/sgml/advanced.sgml,v 1.14 2000/06/14 13:12:52 thomas Exp $
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-->
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1999-08-08 06:21:33 +02:00
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<chapter id="advanced">
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<title>Advanced <productname>Postgres</productname> <acronym>SQL</acronym> Features</title>
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<para>
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Having covered the basics of using
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<productname>Postgres</productname> <acronym>SQL</acronym> to
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access your data, we will now discuss those features of
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<productname>Postgres</productname> that distinguish it from conventional data
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managers. These features include inheritance, time
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travel and non-atomic data values (array- and
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set-valued attributes).
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Examples in this section can also be found in
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<filename>advance.sql</filename> in the tutorial directory.
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(Refer to <xref linkend="QUERY"> for how to use it.)
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</para>
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<sect1>
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<title>Inheritance</title>
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<para>
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Let's create two classes. The capitals class contains
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state capitals which are also cities. Naturally, the
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capitals class should inherit from cities.
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<programlisting>
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CREATE TABLE cities (
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name text,
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population float,
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altitude int -- (in ft)
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);
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CREATE TABLE capitals (
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state char(2)
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) UNDER cities;
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</programlisting>
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In this case, an instance of capitals <firstterm>inherits</firstterm> all
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attributes (name, population, and altitude) from its
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parent, cities. The type of the attribute name is
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<type>text</type>, a native <productname>Postgres</productname>
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type for variable length
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ASCII strings. The type of the attribute population is
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<type>float</type>, a native <productname>Postgres</productname>
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type for double precision
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floating point numbers. State capitals have an extra
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attribute, state, that shows their state.
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In <productname>Postgres</productname>,
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a class can inherit from zero or more other classes,
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and a query can reference either all instances of a
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class or all instances of a class plus all of its
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descendants.
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<note>
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<para>
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The inheritance hierarchy is a directed acyclic graph.
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</para>
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</note>
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</para>
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<para>
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For example, the following query finds the names of all cities,
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including state capitals, that are located at an altitude
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over 500ft, the query is:
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<programlisting>
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SELECT c.name, c.altitude
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FROM cities c
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WHERE c.altitude > 500;
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</programlisting>
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which returns:
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<programlisting>
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+----------+----------+
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|name | altitude |
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+----------+----------+
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|Las Vegas | 2174 |
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+----------+----------+
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|Mariposa | 1953 |
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+----------+----------+
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|Madison | 845 |
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+----------+----------+
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</programlisting>
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</para>
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<para>
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On the other hand, the following query finds
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all the cities, but not capital cities
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that are situated at an attitude of 500ft or higher:
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<programlisting>
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SELECT name, altitude
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FROM ONLY cities
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WHERE altitude > 500;
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+----------+----------+
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|name | altitude |
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+----------+----------+
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|Las Vegas | 2174 |
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+----------+----------+
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|Mariposa | 1953 |
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+----------+----------+
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</programlisting>
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</para>
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<para>
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Here the <quote>ONLY</quote> before cities indicates that the query should
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be run over only cities and not classes below cities in the
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inheritance hierarchy. Many of the commands that we
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have already discussed -- <command>SELECT</command>,
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<command>UPDATE</command> and <command>DELETE</command> --
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support this <quote>ONLY</quote> notation.
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</para>
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<para>
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Deprecated: In previous versions of postgres, the default was not to
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get access to child classes. By experience this was found to be error
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prone. Under the old syntax, to get the sub-classes you append "*"
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to the table name. For example
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<programlisting>
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SELECT * from cities*;
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</programlisting>
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This old behaviour is still available by using a SET command:
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<programlisting>
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SET EXAMINE_SUBCLASS TO on;
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</programlisting>
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</para>
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</sect1>
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<sect1>
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<title>Non-Atomic Values</title>
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<para>
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One of the tenets of the relational model is that the
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attributes of a relation are atomic.
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<productname>Postgres</productname> does not
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have this restriction; attributes can themselves contain
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sub-values that can be accessed from the query
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language. For example, you can create attributes that
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are arrays of base types.
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</para>
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<sect2>
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<title>Arrays</title>
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<para>
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<productname>Postgres</productname> allows attributes of an
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instance to be defined
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as fixed-length or variable-length multi-dimensional
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arrays. Arrays of any base type or user-defined type
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can be created. To illustrate their use, we first create a
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class with arrays of base types.
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<programlisting>
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CREATE TABLE SAL_EMP (
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name text,
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pay_by_quarter int4[],
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schedule text[][]
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);
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</programlisting>
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</para>
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<para>
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The above query will create a class named SAL_EMP with
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a <firstterm>text</firstterm> string (name), a one-dimensional
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array of <firstterm>int4</firstterm>
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(pay_by_quarter), which represents the employee's
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salary by quarter and a two-dimensional array of
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<firstterm>text</firstterm>
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(schedule), which represents the employee's weekly
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schedule. Now we do some <firstterm>INSERTS</firstterm>s;
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note that when
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appending to an array, we enclose the values within
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braces and separate them by commas. If you know
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<firstterm>C</firstterm>,
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this is not unlike the syntax for initializing structures.
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<programlisting>
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INSERT INTO SAL_EMP
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VALUES ('Bill',
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'{10000, 10000, 10000, 10000}',
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'{{"meeting", "lunch"}, {}}');
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INSERT INTO SAL_EMP
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VALUES ('Carol',
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'{20000, 25000, 25000, 25000}',
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'{{"talk", "consult"}, {"meeting"}}');
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</programlisting>
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By default, <productname>Postgres</productname> uses the
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"one-based" numbering
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convention for arrays -- that is, an array of n elements
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starts with array[1] and ends with array[n].
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Now, we can run some queries on SAL_EMP. First, we
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show how to access a single element of an array at a
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time. This query retrieves the names of the employees
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whose pay changed in the second quarter:
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<programlisting>
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SELECT name
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FROM SAL_EMP
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WHERE SAL_EMP.pay_by_quarter[1] <>
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SAL_EMP.pay_by_quarter[2];
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+------+
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|name |
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+------+
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|Carol |
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+------+
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</programlisting>
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</para>
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<para>
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This query retrieves the third quarter pay of all
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employees:
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<programlisting>
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SELECT SAL_EMP.pay_by_quarter[3] FROM SAL_EMP;
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+---------------+
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|pay_by_quarter |
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+---------------+
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|10000 |
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+---------------+
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|25000 |
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+---------------+
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</programlisting>
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</para>
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<para>
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We can also access arbitrary slices of an array, or
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subarrays. This query retrieves the first item on
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Bill's schedule for the first two days of the week.
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<programlisting>
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SELECT SAL_EMP.schedule[1:2][1:1]
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FROM SAL_EMP
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WHERE SAL_EMP.name = 'Bill';
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+-------------------+
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|schedule |
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+-------------------+
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|{{"meeting"},{""}} |
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+-------------------+
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</programlisting>
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</para>
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</sect2>
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</sect1>
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<sect1>
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<title>More Advanced Features</title>
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<para>
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<productname>Postgres</productname> has many features not touched
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upon in this
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tutorial introduction, which has been oriented toward newer users of
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<acronym>SQL</acronym>.
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These are discussed in more detail in both the User's and
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Programmer's Guides.
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</para>
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</sect1>
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1999-08-08 06:21:33 +02:00
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode:sgml
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sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"./reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:("/usr/lib/sgml/catalog")
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sgml-local-ecat-files:nil
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End:
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-->
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