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postgresql
42th chunk of `doc/src/sgml/ref/psql-ref.sgml`
3563c199c2ea493cf20722c9749ff47cfb9e47df3c47e6890000000100000fd5
 <literal>false</literal>, <literal>1</literal>,
        <literal>0</literal>, <literal>on</literal>, <literal>off</literal>,
        <literal>yes</literal>, <literal>no</literal>.  For example,
        <literal>t</literal>, <literal>T</literal>, and <literal>tR</literal>
        will all be considered to be <literal>true</literal>.
        </para>
        <para>
        Expressions that do not properly evaluate to true or false will
        generate a warning and be treated as false.
        </para>
        <para>
        Lines being skipped are parsed normally to identify queries and
        backslash commands, but queries are not sent to the server, and
        backslash commands other than conditionals
        (<command>\if</command>, <command>\elif</command>,
        <command>\else</command>, <command>\endif</command>) are
        ignored.  Conditional commands are checked only for valid nesting.
        Variable references in skipped lines are not expanded, and backquote
        expansion is not performed either.
        </para>
        <para>
        All the backslash commands of a given conditional block must appear in
        the same source file. If EOF is reached on the main input file or an
        <command>\include</command>-ed file before all local
        <command>\if</command>-blocks have been closed,
        then <application>psql</application> will raise an error.
        </para>
        <para>
         Here is an example:
        </para>
<programlisting>
-- check for the existence of two separate records in the database and store
-- the results in separate psql variables
SELECT
    EXISTS(SELECT 1 FROM customer WHERE customer_id = 123) as is_customer,
    EXISTS(SELECT 1 FROM employee WHERE employee_id = 456) as is_employee
\gset
\if :is_customer
    SELECT * FROM customer WHERE customer_id = 123;
\elif :is_employee
    \echo 'is not a customer but is an employee'
    SELECT * FROM employee WHERE employee_id = 456;
\else
    \if yes
        \echo 'not a customer or employee'
    \else
        \echo 'this will never print'
    \endif
\endif
</programlisting>
        </listitem>
      </varlistentry>


      <varlistentry id="app-psql-meta-command-include-relative">
        <term><literal>\ir</literal> or <literal>\include_relative</literal> <replaceable class="parameter">filename</replaceable></term>
        <listitem>
        <para>
        The <literal>\ir</literal> command is similar to <literal>\i</literal>, but resolves
        relative file names differently.  When executing in interactive mode,
        the two commands behave identically.  However, when invoked from a
        script, <literal>\ir</literal> interprets file names relative to the
        directory in which the script is located, rather than the current
        working directory.
        </para>
        </listitem>
      </varlistentry>


      <varlistentry id="app-psql-meta-command-list">
        <term><literal>\l[x+]</literal> or <literal>\list[x+] [ <link linkend="app-psql-patterns"><replaceable class="parameter">pattern</replaceable></link> ]</literal></term>
        <listitem>
        <para>
        List the databases in the server and show their names, owners,
        character set encodings, and access privileges.
        If <replaceable class="parameter">pattern</replaceable> is specified,
        only databases whose names match the pattern are listed.
        If <literal>x</literal> is appended to the command name, the results
        are displayed in expanded mode.
        If <literal>+</literal> is appended to the command name, database
        sizes, default tablespaces, and descriptions are also displayed.
        (Size information is only available for databases that the current
        user can connect to.)
        </para>
        </listitem>
      </varlistentry>


      <varlistentry id="app-psql-meta-command-lo-export">
        <term><literal>\lo_export <replaceable class="parameter">loid</replaceable> <replaceable class="parameter">filename</replaceable></literal></term>
Title: psql Meta-Commands: \if, \ir, \l, \lo_export
Summary
This section describes several psql meta-commands: `\if` (conditional execution blocks), `\ir` or `\include_relative` (similar to `\i` but resolves relative file names relative to the script directory), `\l` or `\list` (lists databases on the server with options for expanded mode and additional information), and `\lo_export` (exports a large object). The \if command includes an example. \ir is only different from \i when executed from a script.