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28th chunk of `doc/src/sgml/plpgsql.sgml`
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     <listitem>
      <para><literal>IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF</literal></para>
     </listitem>
    </itemizedlist>

    and two forms of <command>CASE</command>:
    <itemizedlist>
     <listitem>
      <para><literal>CASE ... WHEN ... THEN ... ELSE ... END CASE</literal></para>
     </listitem>
     <listitem>
      <para><literal>CASE WHEN ... THEN ... ELSE ... END CASE</literal></para>
     </listitem>
    </itemizedlist>
    </para>

    <sect3 id="plpgsql-conditionals-if-then">
     <title><literal>IF-THEN</literal></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN</literal> statements are the simplest form of
        <literal>IF</literal>. The statements between
        <literal>THEN</literal> and <literal>END IF</literal> will be
        executed if the condition is true. Otherwise, they are
        skipped.
       </para>

       <para>
        Example:
<programlisting>
IF v_user_id &lt;&gt; 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;
</programlisting>
       </para>
     </sect3>

     <sect3 id="plpgsql-conditionals-if-then-else">
      <title><literal>IF-THEN-ELSE</literal></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
ELSE
    <replaceable>statements</replaceable>
END IF;
</synopsis>

       <para>
        <literal>IF-THEN-ELSE</literal> statements add to
        <literal>IF-THEN</literal> by letting you specify an
        alternative set of statements that should be executed if the
        condition is not true.  (Note this includes the case where the
        condition evaluates to NULL.)
       </para>

       <para>
        Examples:
<programlisting>
IF parentid IS NULL OR parentid = ''
THEN
    RETURN fullname;
ELSE
    RETURN hp_true_filename(parentid) || '/' || fullname;
END IF;
</programlisting>

<programlisting>
IF v_count &gt; 0 THEN
    INSERT INTO users_count (count) VALUES (v_count);
    RETURN 't';
ELSE
    RETURN 'f';
END IF;
</programlisting>
     </para>
    </sect3>

     <sect3 id="plpgsql-conditionals-if-then-elsif">
      <title><literal>IF-THEN-ELSIF</literal></title>

<synopsis>
IF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
<optional> ELSIF <replaceable>boolean-expression</replaceable> THEN
    <replaceable>statements</replaceable>
    ...
</optional>
</optional>
<optional> ELSE
    <replaceable>statements</replaceable> </optional>
END IF;
</synopsis>

       <para>
        Sometimes there are more than just two alternatives.
        <literal>IF-THEN-ELSIF</literal> provides a convenient
        method of checking several alternatives in turn.
        The <literal>IF</literal> conditions are tested successively
        until the first one that is true is found.  Then the
        associated statement(s) are executed, after which control
        passes to the next statement after <literal>END IF</literal>.
        (Any subsequent <literal>IF</literal> conditions are <emphasis>not</emphasis>
        tested.)  If none of the <literal>IF</literal> conditions is true,
        then the <literal>ELSE</literal> block (if any) is executed.
       </para>

       <para>
        Here is an example:

<programlisting>
IF number = 0 THEN
    result := 'zero';
ELSIF number &gt; 0 THEN
    result := 'positive';
ELSIF number &lt; 0 THEN
    result := 'negative';
ELSE
    -- hmm, the only other possibility is that number is null
    result := 'NULL';
END IF;
</programlisting>
       </para>

       <para>
        The key word <literal>ELSIF</literal> can also be spelled
        <literal>ELSEIF</literal>.
       </para>

       <para>
        An alternative way of accomplishing the same task is to nest
Title: IF-THEN, IF-THEN-ELSE, and IF-THEN-ELSIF Conditional Statements
Summary
This section details the three forms of IF conditional statements in PL/pgSQL. The IF-THEN statement executes statements between THEN and END IF when the condition is true. The IF-THEN-ELSE statement extends IF-THEN by providing an alternative set of statements to execute when the condition is false (or NULL). The IF-THEN-ELSIF statement offers a method to check several alternatives in sequence; conditions are tested until a true condition is found, after which the corresponding statements are executed, and control passes to the statement after END IF. If no condition is true, the ELSE block (if present) is executed.