</varlistentry>

<varlistentry>

<term><literal>\if</literal> <replaceable class="parameter">expression</replaceable></term>

<term><literal>\elif</literal> <replaceable class="parameter">expression</replaceable></term>

<term><literal>\else</literal></term>

<term><literal>\endif</literal></term>

<listitem>

<para>

This group of commands implements nestable conditional blocks.

A conditional block must begin with an <command>\if</command> and end

with an <command>\endif</command>. In between there may be any number

of <command>\elif</command> clauses, which may optionally be followed

by a single <command>\else</command> clause. Ordinary queries and

other types of backslash commands may (and usually do) appear between

the commands forming a conditional block.

</para>

<para>

The <command>\if</command> and <command>\elif</command> commands read

their argument(s) and evaluate them as a boolean expression. If the

expression yields <literal>true</literal> then processing continues

normally; otherwise, lines are skipped until a

matching <command>\elif</command>, <command>\else</command>,

or <command>\endif</command> is reached. Once

an <command>\if</command> or <command>\elif</command> test has

succeeded, the arguments of later <command>\elif</command> commands in

the same block are not evaluated but are treated as false. Lines

following an <command>\else</command> are processed only if no earlier

matching <command>\if</command> or <command>\elif</command> succeeded.

</para>

<para>

The <replaceable class="parameter">expression</replaceable> argument

of an <command>\if</command> or <command>\elif</command> command

is subject to variable interpolation and backquote expansion, just

like any other backslash command argument. After that it is evaluated

like the value of an on/off option variable. So a valid value

is any unambiguous case-insensitive match for one of:

<literal>true</literal>, <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</> 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>

<term><literal>\l[+]</literal> or <literal>\list[+] [ <link linkend="APP-PSQL-patterns"><replaceable class="parameter">pattern</replaceable></link> ]</literal></term>

<listitem>

<listitem>

<para>

In prompt 1 normally <literal>=</literal>,

but <literal>^</literal> if in single-line mode,

but <literal>@</literal> if the session is in an inactive branch of a

conditional block, or <literal>^</literal> if in single-line mode,

or <literal>!</literal> if the session is disconnected from the

database (which can happen if <command>\connect</command> fails).

In prompt 2 <literal>%R</literal> is replaced by a character that

overrideCPPFLAGS := -I. -I$(srcdir) -I$(libpq_srcdir)$(CPPFLAGS)

LDFLAGS += -L$(top_builddir)/src/fe_utils -lpgfeutils -lpq

OBJS=command.o common.ohelp.oinput.ostringutils.o mainloop.o copy.o\

startup.oprompt.ovariables.o large_obj.odescribe.o\

crosstabview.otab-complete.o\

sql_help.opsqlscanslash.o\

OBJS=command.o common.oconditional.ocopy.ocrosstabview.o\

describe.ohelp.oinput.o large_obj.omainloop.o\

prompt.opsqlscanslash.o sql_help.o startup.o stringutils.o\

tab-complete.ovariables.o\