~ubuntu-branches/ubuntu/trusty/postgresql-9.3/trusty-proposed : contents of doc/src/sgml/html/plpgsql-structure.html at revision 22

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>Chapter 40. <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> - <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Procedural Language</TD
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><DIV
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><H1
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><A
NAME="PLPGSQL-STRUCTURE"
>40.2. Structure of <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
></A
></H1
><P
>   <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> is a block-structured language.
   The complete text of a function definition must be a
   <I
CLASS="FIRSTTERM"
>block</I
>. A block is defined as:

</P><PRE
CLASS="SYNOPSIS"
>[<SPAN
CLASS="OPTIONAL"
> &lt;&lt;<TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
>&gt;&gt; </SPAN
>]
[<SPAN
CLASS="OPTIONAL"
> DECLARE
    <TT
CLASS="REPLACEABLE"
><I
>declarations</I
></TT
> </SPAN
>]
BEGIN
    <TT
CLASS="REPLACEABLE"
><I
>statements</I
></TT
>
END [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> </SPAN
>];</PRE
><P>
    </P
><P
>     Each declaration and each statement within a block is terminated
     by a semicolon.  A block that appears within another block must
     have a semicolon after <TT
CLASS="LITERAL"
>END</TT
>, as shown above;
     however the final <TT
CLASS="LITERAL"
>END</TT
> that
     concludes a function body does not require a semicolon.
    </P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>      A common mistake is to write a semicolon immediately after
      <TT
CLASS="LITERAL"
>BEGIN</TT
>.  This is incorrect and will result in a syntax error.
     </P
></BLOCKQUOTE
></DIV
><P
>     A <TT
CLASS="REPLACEABLE"
><I
>label</I
></TT
> is only needed if you want to
     identify the block for use
     in an <TT
CLASS="LITERAL"
>EXIT</TT
> statement, or to qualify the names of the
     variables declared in the block.  If a label is given after
     <TT
CLASS="LITERAL"
>END</TT
>, it must match the label at the block's beginning.
    </P
><P
>     All key words are case-insensitive.
     Identifiers are implicitly converted to lower case
     unless double-quoted, just as they are in ordinary SQL commands.
    </P
><P
>     Comments work the same way in <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> code as in
     ordinary SQL.  A double dash (<TT
CLASS="LITERAL"
>--</TT
>) starts a comment
     that extends to the end of the line. A <TT
CLASS="LITERAL"
>/*</TT
> starts a
     block comment that extends to the matching occurrence of
     <TT
CLASS="LITERAL"
>*/</TT
>.  Block comments nest.
    </P
><P
>     Any statement in the statement section of a block
     can be a <I
CLASS="FIRSTTERM"
>subblock</I
>.  Subblocks can be used for
     logical grouping or to localize variables to a small group
     of statements.  Variables declared in a subblock mask any
     similarly-named variables of outer blocks for the duration
     of the subblock; but you can access the outer variables anyway
     if you qualify their names with their block's label. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION somefunc() RETURNS integer AS $$
&lt;&lt; outerblock &gt;&gt;
DECLARE
    quantity integer := 30;
BEGIN
    RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 30
    quantity := 50;
    --
    -- Create a subblock
    --
    DECLARE
        quantity integer := 80;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 80
        RAISE NOTICE 'Outer quantity here is %', outerblock.quantity;  -- Prints 50
    END;

    RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 50

    RETURN quantity;
END;
$$ LANGUAGE plpgsql;</PRE
><P>
    </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>      There is actually a hidden <SPAN
CLASS="QUOTE"
>"outer block"</SPAN
> surrounding the body
      of any <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> function.  This block provides the
      declarations of the function's parameters (if any), as well as some
      special variables such as <TT
CLASS="LITERAL"
>FOUND</TT
> (see
      <A
HREF="plpgsql-statements.html#PLPGSQL-STATEMENTS-DIAGNOSTICS"
>Section 40.5.5</A
>).  The outer block is
      labeled with the function's name, meaning that parameters and special
      variables can be qualified with the function's name.
     </P
></BLOCKQUOTE
></DIV
><P
>     It is important not to confuse the use of
     <TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>END</TT
> for grouping statements in
     <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> with the similarly-named SQL commands
     for transaction
     control.  <SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
>'s <TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>END</TT
>
     are only for grouping; they do not start or end a transaction.
     Functions and trigger procedures are always executed within a transaction
     established by an outer query &mdash; they cannot start or commit that
     transaction, since there would be no context for them to execute in.
     However, a block containing an <TT
CLASS="LITERAL"
>EXCEPTION</TT
> clause effectively
     forms a subtransaction that can be rolled back without affecting the
     outer transaction.  For more about that see <A
HREF="plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING"
>Section 40.6.6</A
>.
    </P
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