HSQLDB example source code file (texttables.xml)

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The HSQLDB texttables.xml source code

<?xml version="1.0" encoding="UTF-8"?>
<!-- $Id: texttables.xml,v 1.14 2007/05/30 18:34:46 fredt Exp $ -->
<chapter id="texttables-chapter">
  <title id="texttables-title">Text Tables

  <subtitle>Text Tables as a Standard Feature of Hsqldb

  <chapterinfo>
    <authorgroup>
      <author>
        <firstname>Bob

        <surname>Preston

        <affiliation>
          <orgname>HSQLDB Development Group
        </affiliation>
      </author>

      <author>
        <firstname>Fred

        <surname>Toussi

        <affiliation>
          <orgname>HSQLDB Development Group
        </affiliation>

        <email>ft@cluedup.com
      </author>
    </authorgroup>

    <edition>$Revision: 1.14 $

    <pubdate>$Date: 2007/05/30 18:34:46 $

    <keywordset>
      <keyword>Text

      <keyword>Tables
    </keywordset>

    <legalnotice>
      <para>Copyright 2002-2005 Bob Preston and Fred Toussi. Permission is
      granted to distribute this document without any alteration under the
      terms of the HSQLDB license. Additional permission is granted to the
      HSQLDB Development Group to distribute this document with or without
      alterations under the terms of the HSQLDB license.</para>
    </legalnotice>
  </chapterinfo>

  <para>Text Table support for HSQLDB was originally developed by Bob Preston
  independently from the Project. Subsequently Bob joined the Project and
  incorporated this feature into version 1.7.0, with a number of enhancements,
  especially the use of conventional SQL commands for specifying the files
  used for Text Tables.</para>

  <para>In a nutshell, Text Tables are CSV or other delimited files treated as
  SQL tables. Any ordinary CSV or other delimited file can be used. The full
  range of SQL queries can be performed on these files, including SELECT,
  INSERT, UPDATE and DELETE. Indexes and unique constraints can be set up, and
  foreign key constraints can be used to enforce referential integrity between
  Text Tables themselves or with conventional tables.</para>

  <para>HSQLDB with Text Table support is the only comprehensive solution that
  employs the power of SQL and the universal reach of JDBC to handle data
  stored in text files and will have wide-ranging use way beyond the currently
  established Java realm of HSQLDB.</para>

  <orderedlist>
    <title>Goals of the Implementation

    <listitem>
      <para>We aimed to finalise the DDL for Text Tables so that future
      releases of HSQLDB use the same DDL scripts.</para>
    </listitem>

    <listitem>
      <para>We aimed to support Text Tables as GLOBAL TEMPORARY or GLOBAL BASE
      tables in the SQL domain.</para>
    </listitem>
  </orderedlist>

  <section>
    <title>The Implementation

    <section>
      <title>Definition of Tables

      <para>Text Tables are defined similarly to conventional tables with the
      added TEXT keyword:</para>

      <programlisting>
    CREATE TEXT TABLE <tablename> (<column definition> [<constraint definition>])</programlisting>

      <para>In addition, a SET command specifies the file and the separator
      character that the Text table uses:</para>

      <programlisting>
    SET TABLE <tablename> SOURCE <quoted_filename_and_options> [DESC]</programlisting>

      <para>Text Tables cannot be created in memory-only databases (databases
      that have no script file).</para>
    </section>

    <section>
      <title>Scope and Reassignment

      <itemizedlist>
        <listitem>
          <para>A Text table without a file assigned to it is READ ONLY and
          EMPTY.</para>
        </listitem>

        <listitem>
          <para>A Temporary Text table has the scope and the lifetime of the
          SQL session (a JDBC Connection).</para>
        </listitem>

        <listitem>
          <para>Reassigning a Text Table definition to a new file has
          implications in the following areas:</para>

          <orderedlist>
            <listitem>
              <para>The user is required to be an administrator.
            </listitem>

            <listitem>
              <para>Existing transactions are committed at this point.
            </listitem>

            <listitem>
              <para>Constraints, including foreign keys referencing this
              table, are kept intact. It is the responsibility of the
              administrator to ensure their integrity.</para>
            </listitem>
          </orderedlist>

          <para>From version 1.7.2 the new source file is scanned and indexes
          are built when it is assigned to the table. At this point any
          violation of NOT NULL, UNIQUE or PRIMARY KEY constrainst are caught
          and the assignment is aborted. However, foreign key constraints are
          not checked at the time of assignment or reassignment of the source
          file.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Null Values in Columns of Text Tables

      <para>This has changed since 1.7.2 to support both null values and empty
      strings.</para>

      <itemizedlist>
        <listitem>
          <para>Empty fields are treated as NULL. These are fields where there
          is nothing or just spaces between the separators.</para>
        </listitem>

        <listitem>
          <para>Quoted empty strings are treated as empty strings.
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Configuration

      <para>The default field separator is a comma (,). A different field
      separator can be specified within the SET TABLE SOURCE statement. For
      example, to change the field separator for the table mytable to a
      vertical bar, place the following in the SET TABLE SOURCE statement, for
      example:</para>

      <informalexample>
        <programlisting>
    SET TABLE mytable SOURCE "myfile;fs=|"</programlisting>
      </informalexample>

      <para>Since HSQLDB treats CHAR's, VARCHARs, and LONGVARCHARs the same,
      the ability to assign different separators to the latter two is
      provided. When a different separator is assigned to a VARCHAR or
      LONGVARCHAR field, it will terminate any CSV field of that type. For
      example, if the first field is CHAR, and the second field LONGVARCHAR,
      and the separator fs has been defined as the pipe (|) and vs as the
      period (.) then the data in the CSV file for a row will look
      like:</para>

      <screen>
    First field data|Second field data.Third field data</screen>

      <para>The following example shows how to change the default separator to
      the pipe (|), VARCHAR separator to the period (.) and the LONGVARCHAR
      separator to the tilde (~). Place the following within the SET TABLE
      SOURCE statement, for example:</para>

      <informalexample>
        <programlisting>
    SET TABLE mytable SOURCE "myfile;fs=|;vs=.;lvs=~"</programlisting>
      </informalexample>

      <para>HSQLDB also recognises the following special indicators for
      separators:</para>

      <variablelist>
        <title>special indicators for separators

        <varlistentry>
          <term>\semi

          <listitem>
            <para>semicolon
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\quote

          <listitem>
            <para>qoute
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\space

          <listitem>
            <para>space character
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\apos

          <listitem>
            <para>apostrophe
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\n

          <listitem>
            <para>newline - Used as an end anchor (like $ in regular
            expressions)</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\r

          <listitem>
            <para>carriage return
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\t

          <listitem>
            <para>tab
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\\

          <listitem>
            <para>backslash
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>\u####

          <listitem>
            <para>a Unicode character specified in hexadecimal
          </listitem>
        </varlistentry>
      </variablelist>

      <para>Furthermore, HSQLDB provides csv file support with three
      additional boolean options: <literal>ignore_first,
      <literal>quoted and all_quoted. The
      <literal>ignore_first option (default false) tells HSQLDB to
      ignore the first line in a file. This option is used when the first line
      of the file contains column headings. The <literal>all_quoted
      option (default false) tells the program that it should use quotes
      around all character fields when writing to the source file. The
      <literal>quoted option (default true) uses quotes only when
      necessary to distinguish a field that contains the separator character.
      It can be set to false to prevent the use of quoting altogether and
      treat quote characters as normal characters. These options may be
      specified within the <literal>SET TABLE SOURCE
      statement:</para>

      <programlisting>
    SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true"</programlisting>

      <para>When the default options all_quoted=
      <literal>false and quoted=true are in
      force, fields that are written to a line of the csv file will be quoted
      only if they contain the separator or the quote character. The quote
      character is doubled when used inside a string. When
      <literal>all_quoted=false and quoted=false
      the quote character is not doubled. With this option, it is not possible
      to insert any string containing the separator into the table, as it
      would become impossible to distinguish from a separator. While reading
      an existing data source file, the program treats each individual field
      separately. It determines that a field is quoted only if the first
      character is the quote character. It interprets the rest of the field on
      this basis.</para>

      <para>The character encoding for the source file is ASCII
      </literal>by default. To support UNICODE or source files preprared with
      different encodings this can be changed to <literal>UTF-8 or
      any other encoding. The default is <literal>encoding=ASCII and
      the option <literal>encoding=UTF-8 or other supported
      encodings can be used.</para>

      <para>Finally, HSQLDB provides the ability to read a text file from the
      bottom up and making them READ ONLY, by placing the keyword "DESC" at
      the end of the SET TABLE SOURCE statement:</para>

      <programlisting>
    SET TABLE mytable SOURCE "myfile" DESC</programlisting>

      <para>This feature provides functionality similar to the Unix tail
      command, by re-reading the file each time a select is executed. Using
      this feature sets the table to read-only mode. Afterwards, it will no
      longer be possible to change the read-only status with <literal>SET
      TABLE <tablename> READONLY TRUE</literal>.

      <para>Text table source files are cached in memory. The maximum number
      of rows of data that are in memory at any time is controlled by the
      <literal>textdb.cache_scale  property. The default value for
      <literal>textdb.cache_scale is 10 and can be changed by
      setting the property in the .properties file for the database. The
      number of rows in memory is calculated as 3*(2**scale), which translates
      to 3072 rows for the default textdb.cache_scale setting (10). The
      property can also be set for individual text tables:</para>

      <programlisting>
    SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true;cache_scale=12"</programlisting>

    </section>
    <section id='disconnecting_text_tables'>
      <title>Disconnecting Text Tables

      <para>The following describes behaviour present in 1.8.0.8 and later.

      <para>Text tables may be disconnected from their underlying data source, i.e.
      the text file.</para>

      <para>You can explicitly disconnect a text table from its file by issuing the
      following statement:
      <programlisting>  SET TABLE mytable SOURCE OFF
      </para>

      <para>Subsequently, mytable will be empty and read-only. However, the data
      source description will be preserved, and the table can be re-connected to it with
      <programlisting>  SET TABLE mytable SOURCE ON
      </para>

      <para>When a database is opened, if the source file for an existing text table is missing
      the table remains disconnected from its data source, but the source sescription is preserved.
      This allows the missing source file to be added to the directory and the table re-connected to it
      with the above command.
      </para>
    </section>
  </section>

  <!-- Blaine adding section here because otherwise the resultant documents
         looke exactly like the following is all part of the Implementation
         section. -->

  <section>
    <title>Text File Issues

    <itemizedlist>
      <title>Text File Issues

      <listitem>
        <para>File locations are restricted to below the directory that
        contains the database, unless the textdb.allow_full_path property is
        set true in the database properties file.</para>
      </listitem>

      <listitem>
        <para>Blank lines are allowed anywhere in the text file, and are
        ignored.</para>
      </listitem>

      <listitem>
        <para>The file location for a text table created with

        <programlisting>
    SELECT <select list> INTO TEXT <tablename> FROM</programlisting>

        <para>is the directory that contains the database and the file name is
        based on the table name. The table name is converted into the file
        name by replacing all the non-alphanumeric characters with the
        underscore character, conversion into lowercase, and adding the ".csv"
        suffix.</para>
      </listitem>

      <listitem>
        <para>It is possible to define a primay key or
        identity column for text tables.</para>
      </listitem>

      <listitem>
        <para>When a table source file is used with the
        ignore_first=true </literal>option, the first, ignored line is
        replaced with a blank line after a SHUTDOWN COMPACT.</para>
      </listitem>

      <listitem>
        <para>An existing table source file may include CHARACTER fields that
        do not begin with the quote character but contain instances of the
        quote character. These fields are read as literal strings.
        Alternatively, if any field begins with the quote character, then it
        is interpreted as a quoted string that should end with the quote
        character and any instances of the quote character within the string
        is doubled. When any field containing the quote character or the
        separator is written out to the source file by the program, the field
        is enclosed in quote character and any instance of the quote character
        inside the field is doubled.</para>
      </listitem>

      <listitem>
        <para>Inserts or updates of CHARACTER type field values are allowed
        with strings that contains the linefeed or the carriage return
        character. This feature is disabled when both quoted and all_quoted
        properties are false.</para>
      </listitem>
      <listitem>
        <para>ALTER TABLE commands that add or drop columns are not supported 
        with non-empty text tables.</para>
      </listitem>
    </itemizedlist>
  </section>

  <!-- See my comment for the previous section. -->

  <section>
    <title>Text File Global Properties

    <itemizedlist>
      <title>Complete list of supported global properties in *.properties
      files</title>

      <listitem>
        <para>
          <literal>textdb.fs
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.lvs
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.quoted
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.all_quoted
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.ignore_first
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.encoding
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.cache_scale
        </para>
      </listitem>

      <listitem>
        <para>
          <literal>textdb.allow_full_path
        </para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Importing a Text Table file in to a Traditional (non-Text Table)
    Table</title>

    <titleabbrev>Importing from a Text Table file

    <simpara>The directory src/org/hsqldb/sample in your
    HSQLDB distibution contains a file named
    <filename>load_binding_lu.sql. This is a working SQL file which
    imports a pipe-delimited text file from the database's file directory into
    an existing normal table. You can edit a copy of this file and use it
    directly with <link linkend="sqltool-chapter">SqlTool, or you can
    use the SQL therein as a model (using any SQL client at all).</simpara>

    <!--
        <example>