query.sgml

PostgreSQL7.4.6 for Linux

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 <chapter id="tutorial-sql">
  <title>The <acronym>SQL</acronym> Language</title>

  <sect1 id="tutorial-sql-intro">
   <title>Introduction</title>

   <para>
    This chapter provides an overview of how to use
    <acronym>SQL</acronym> to perform simple operations.  This
    tutorial is only intended to give you an introduction and is in no
    way a complete tutorial on <acronym>SQL</acronym>.  Numerous books
    have been written on <acronym>SQL</acronym>, including <xref
    linkend="MELT93"> and <xref linkend="DATE97">.
    You should be aware that some <productname>PostgreSQL</productname>
    language features are extensions to the standard.
   </para>

   <para>
    In the examples that follow, we assume that you have created a
    database named <literal>mydb</literal>, as described in the previous
    chapter, and have started <application>psql</application>.
   </para>

   <para>
    Examples in this manual can also be found in the
    <productname>PostgreSQL</productname> source distribution
    in the directory <filename>src/tutorial/</filename>.  Refer to the
    <filename>README</filename> file in that directory for how to use
    them.  To start the tutorial, do the following:

<screen>
<prompt>$</prompt> <userinput>cd <replaceable>....</replaceable>/src/tutorial</userinput>
<prompt>$</prompt> <userinput>psql -s mydb</userinput>
<computeroutput>
...
</computeroutput>

<prompt>mydb=&gt;</prompt> <userinput>\i basics.sql</userinput>
</screen>

    The <literal>\i</literal> command reads in commands from the
    specified file. The <literal>-s</literal> option puts you in
    single step mode which pauses before sending each statement to the
    server.  The commands used in this section are in the file
    <filename>basics.sql</filename>.
   </para>
  </sect1>


  <sect1 id="tutorial-concepts">
   <title>Concepts</title>

   <para>
    <indexterm><primary>relational database</primary></indexterm>
    <indexterm><primary>hierarchical database</primary></indexterm>
    <indexterm><primary>object-oriented database</primary></indexterm>
    <indexterm><primary>relation</primary></indexterm>
    <indexterm><primary>table</primary></indexterm>

    <productname>PostgreSQL</productname> is a <firstterm>relational
    database management system</firstterm> (<acronym>RDBMS</acronym>).
    That means it is a system for managing data stored in
    <firstterm>relations</firstterm>.  Relation is essentially a
    mathematical term for <firstterm>table</firstterm>.  The notion of
    storing data in tables is so commonplace today that it might
    seem inherently obvious, but there are a number of other ways of
    organizing databases.  Files and directories on Unix-like
    operating systems form an example of a hierarchical database.  A
    more modern development is the object-oriented database.
   </para>

   <para>
    <indexterm><primary>row</primary></indexterm>
    <indexterm><primary>column</primary></indexterm>

    Each table is a named collection of <firstterm>rows</firstterm>.
    Each row of a given table has the same set of named
    <firstterm>columns</firstterm>,
    and each column is of a specific data type.  Whereas columns have
    a fixed order in each row, it is important to remember that SQL
    does not guarantee the order of the rows within the table in any
    way (although they can be explicitly sorted for display).
   </para>

   <para>
    <indexterm><primary>database cluster</primary></indexterm>
    <indexterm><primary>cluster</primary><secondary>of databases</secondary><see>database cluster</see></indexterm>

    Tables are grouped into databases, and a collection of databases
    managed by a single <productname>PostgreSQL</productname> server
    instance constitutes a database <firstterm>cluster</firstterm>.
   </para>
  </sect1>


  <sect1 id="tutorial-table">
   <title>Creating a New Table</title>

   <indexterm zone="tutorial-table">
    <primary>CREATE TABLE</primary>
   </indexterm>

   <para>
    You  can  create  a  new  table by specifying the table
    name, along with all column names and their types:

<programlisting>
CREATE TABLE weather (
    city            varchar(80),
    temp_lo         int,           -- low temperature
    temp_hi         int,           -- high temperature
    prcp            real,          -- precipitation
    date            date
);
</programlisting>

    You can enter this into <command>psql</command> with the line
    breaks.  <command>psql</command> will recognize that the command
    is not terminated until the semicolon.
   </para>

   <para>
    White space (i.e., spaces, tabs, and newlines) may be used freely
    in SQL commands.  That means you can type the command aligned
    differently than above, or even all on one line.  Two dashes
    (<quote><literal>--</literal></quote>) introduce comments.
    Whatever follows them is ignored up to the end of the line.  SQL
    is case insensitive about key words and identifiers, except
    when identifiers are double-quoted to preserve the case (not done
    above).
   </para>

   <para>
    <type>varchar(80)</type> specifies a data type that can store
    arbitrary character strings up to 80 characters in length.
    <type>int</type> is the normal integer type.  <type>real</type> is
    a type for storing single precision floating-point numbers.
    <type>date</type> should be self-explanatory.  (Yes, the column of
    type <type>date</type> is also named <literal>date</literal>.
    This may be convenient or confusing -- you choose.)
   </para>

   <para>
    <productname>PostgreSQL</productname> supports the usual
    <acronym>SQL</acronym> types <type>int</type>,
    <type>smallint</type>, <type>real</type>, <type>double
    precision</type>, <type>char(<replaceable>N</>)</type>,
    <type>varchar(<replaceable>N</>)</type>, <type>date</type>,
    <type>time</type>, <type>timestamp</type>, and
    <type>interval</type>, as well as other types of general utility
    and a rich set of geometric types.
    <productname>PostgreSQL</productname> can be customized with an
    arbitrary number of user-defined data types.  Consequently, type
    names are not syntactical key words, except where required to
    support special cases in the <acronym>SQL</acronym> standard.
   </para>

   <para>
    The second example will store cities and their associated
    geographical location:
<programlisting>
CREATE TABLE cities (
    name            varchar(80),
    location        point
);
</programlisting>
    The <type>point</type> type is an example of a
    <productname>PostgreSQL</productname>-specific data type.
   </para>

   <para>
    <indexterm>
     <primary>DROP TABLE</primary>
    </indexterm>

    Finally, it should be mentioned that if you don't need a table any
    longer or want to recreate it differently you can remove it using
    the following command:
<synopsis>
DROP TABLE <replaceable>tablename</replaceable>;
</synopsis>
   </para>
  </sect1>


  <sect1 id="tutorial-populate">
   <title>Populating a Table With Rows</title>

   <indexterm zone="tutorial-populate">
    <primary>INSERT</primary>
   </indexterm>

   <para>
    The <command>INSERT</command> statement is used to populate a table  with
    rows:

<programlisting>
INSERT INTO weather VALUES ('San Francisco', 46, 50, 0.25, '1994-11-27');
</programlisting>

    Note that all data types use rather obvious input formats.
    Constants that are not simple numeric values usually must be
    surrounded by single quotes (<literal>'</>), as in the example.
    The
    <type>date</type> type is actually quite flexible in what it
    accepts, but for this tutorial we will stick to the unambiguous
    format shown here.
   </para>

   <para>
    The <type>point</type> type requires a coordinate pair as input,
    as shown here:
<programlisting>
INSERT INTO cities VALUES ('San Francisco', '(-194.0, 53.0)');
</programlisting>
   </para>

   <para>
    The syntax used so far requires you to remember the order of the
    columns.  An alternative syntax allows you to list the columns
    explicitly:
<programlisting>
INSERT INTO weather (city, temp_lo, temp_hi, prcp, date)
    VALUES ('San Francisco', 43, 57, 0.0, '1994-11-29');
</programlisting>
    You can list the columns in a different order if you wish or
    even omit some columns, e.g., if the precipitation is unknown:
<programlisting>
INSERT INTO weather (date, city, temp_hi, temp_lo)
    VALUES ('1994-11-29', 'Hayward', 54, 37);
</programlisting>
    Many developers consider explicitly listing the columns better
    style than relying on the order implicitly.
   </para>

   <para>
    Please enter all the commands shown above so you have some data to
    work with in the following sections.
   </para>

   <para>
    <indexterm>
     <primary>COPY</primary>
    </indexterm>

    You could also have used <command>COPY</command> to load large
    amounts of data from flat-text files.  This is usually faster
    because the <command>COPY</command> command is optimized for this
    application while allowing less flexibility than
    <command>INSERT</command>.  An example would be:

<programlisting>
COPY weather FROM '/home/user/weather.txt';
</programlisting>

    where the file name for the source file must be available to the
    backend server machine, not the client, since the backend server
    reads the file directly.  You can read more about the
    <command>COPY</command> command in <xref linkend="sql-copy">.
   </para>
  </sect1>


  <sect1 id="tutorial-select">
   <title>Querying a Table</title>

   <para>
    <indexterm><primary>query</primary></indexterm>
    <indexterm><primary>SELECT</primary></indexterm>

    To retrieve data from a table, the table is
    <firstterm>queried</firstterm>.  An <acronym>SQL</acronym>
    <command>SELECT</command> statement is used to do this.  The
    statement is divided into a select list (the part that lists the
    columns to be returned), a table list (the part that lists the
    tables from which to retrieve the data), and an optional
    qualification (the part that specifies any restrictions).  For
    example, to retrieve all the rows of table
    <classname>weather</classname>, type:
<programlisting>
SELECT * FROM weather;
</programlisting>
    (here <literal>*</literal> means <quote>all columns</quote>) and
    the output should be:
<screen>
     city      | temp_lo | temp_hi | prcp |    date
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
 San Francisco |      43 |      57 |    0 | 1994-11-29
 Hayward       |      37 |      54 |      | 1994-11-29
(3 rows)
</screen>
   </para>

   <para>
    You may specify any arbitrary expressions in the select list.  For 
    example, you can do:
<programlisting>
SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;
</programlisting>
    This should give:
<screen>
     city      | temp_avg |    date
---------------+----------+------------
 San Francisco |       48 | 1994-11-27
 San Francisco |       50 | 1994-11-29
 Hayward       |       45 | 1994-11-29
(3 rows)
</screen>
    Notice how the <literal>AS</literal> clause is used to relabel the
    output column.  (It is optional.)
   </para>

   <para>
    Arbitrary Boolean operators (<literal>AND</literal>,
    <literal>OR</literal>, and <literal>NOT</literal>) are allowed in
    the qualification of a query.  For example, the following
    retrieves the weather of San Francisco on rainy days:

<programlisting>
SELECT * FROM weather
    WHERE city = 'San Francisco'
    AND prcp > 0.0;
</programlisting>
    Result:
<screen>
     city      | temp_lo | temp_hi | prcp |    date
---------------+---------+---------+------+------------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27
(1 row)
</screen>
   </para>

   <para>
    <indexterm><primary>ORDER BY</primary></indexterm>
    <indexterm><primary>DISTINCT</primary></indexterm>
    <indexterm><primary>duplicate</primary></indexterm>

    As a final note, you can request that the results of a query can
    be returned in sorted order or with duplicate rows removed:

<programlisting>
SELECT DISTINCT city
    FROM weather
    ORDER BY city;
</programlisting>

<screen>
     city
---------------
 Hayward
 San Francisco
(2 rows)
</screen>

    <literal>DISTINCT</literal> and <literal>ORDER BY</literal> can be
    used separately, of course.
   </para>
  </sect1>


  <sect1 id="tutorial-join">
   <title>Joins Between Tables</title>

   <indexterm zone="tutorial-join">
    <primary>join</primary>
   </indexterm>

   <para>
    Thus far, our queries have only accessed one table at a time.
    Queries can access multiple tables at once, or access the same
    table in such a way that multiple rows of the table are being
    processed at the same time.  A query that accesses multiple rows
    of the same or different tables at one time is called a
    <firstterm>join</firstterm> query.  As an example, say you wish to
    list all the weather records together with the location of the
    associated city.  To do that, we need to compare the city column of
    each row of the weather table with the name column of all rows in
    the cities table, and select the pairs of rows where these values match.
    <note>
     <para>
      This  is only a conceptual model.  The actual join may
      be performed in a more efficient manner, but this is invisible
      to the user.
     </para>
    </note>
    This would be accomplished by the following query:

<programlisting>
SELECT *
    FROM weather, cities
    WHERE city = name;
</programlisting>

<screen>
     city      | temp_lo | temp_hi | prcp |    date    |     name      | location
---------------+---------+---------+------+------------+---------------+-----------
 San Francisco |      46 |      50 | 0.25 | 1994-11-27 | San Francisco | (-194,53)
 San Francisco |      43 |      57 |    0 | 1994-11-29 | San Francisco | (-194,53)
(2 rows)
</screen>

   </para>

   <para>
    Observe two things about the result set:
    <itemizedlist>
     <listitem>
      <para>
       There is no result row for the city of Hayward.  This is