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>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-COMMANDS"
>33.3. Running SQL Commands</A
></H1
><P
> Any SQL command can be run from within an embedded SQL application.
Below are some examples of how to do that.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-EXECUTING"
>33.3.1. Executing SQL Statements</A
></H2
><P
> Creating a table:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL CREATE TABLE foo (number integer, ascii char(16));
EXEC SQL CREATE UNIQUE INDEX num1 ON foo(number);
EXEC SQL COMMIT;</PRE
><P>
</P
><P
> Inserting rows:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL INSERT INTO foo (number, ascii) VALUES (9999, 'doodad');
EXEC SQL COMMIT;</PRE
><P>
</P
><P
> Deleting rows:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DELETE FROM foo WHERE number = 9999;
EXEC SQL COMMIT;</PRE
><P>
</P
><P
> Updates:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL UPDATE foo
SET ascii = 'foobar'
WHERE number = 9999;
EXEC SQL COMMIT;</PRE
><P>
</P
><P
> <TT
CLASS="LITERAL"
>SELECT</TT
> statements that return a single result
row can also be executed using
<TT
CLASS="LITERAL"
>EXEC SQL</TT
> directly. To handle result sets with
multiple rows, an application has to use a cursor;
see <A
HREF="ecpg-commands.html#ECPG-CURSORS"
>Section 33.3.2</A
> below. (As a special case, an
application can fetch multiple rows at once into an array host
variable; see <A
HREF="ecpg-variables.html#ECPG-VARIABLES-ARRAYS"
>Section 33.4.4.3.1</A
>.)
</P
><P
> Single-row select:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL SELECT foo INTO :FooBar FROM table1 WHERE ascii = 'doodad';</PRE
><P>
</P
><P
> Also, a configuration parameter can be retrieved with the
<TT
CLASS="LITERAL"
>SHOW</TT
> command:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL SHOW search_path INTO :var;</PRE
><P>
</P
><P
> The tokens of the form
<TT
CLASS="LITERAL"
>:<TT
CLASS="REPLACEABLE"
><I
>something</I
></TT
></TT
> are
<I
CLASS="FIRSTTERM"
>host variables</I
>, that is, they refer to
variables in the C program. They are explained in <A
HREF="ecpg-variables.html"
>Section 33.4</A
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CURSORS"
>33.3.2. Using Cursors</A
></H2
><P
> To retrieve a result set holding multiple rows, an application has
to declare a cursor and fetch each row from the cursor. The steps
to use a cursor are the following: declare a cursor, open it, fetch
a row from the cursor, repeat, and finally close it.
</P
><P
> Select using cursors:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DECLARE foo_bar CURSOR FOR
SELECT number, ascii FROM foo
ORDER BY ascii;
EXEC SQL OPEN foo_bar;
EXEC SQL FETCH foo_bar INTO :FooBar, DooDad;
...
EXEC SQL CLOSE foo_bar;
EXEC SQL COMMIT;</PRE
><P>
</P
><P
> For more details about declaration of the cursor,
see <A
HREF="ecpg-sql-declare.html"
>DECLARE</A
>, and
see <A
HREF="sql-fetch.html"
>FETCH</A
> for <TT
CLASS="LITERAL"
>FETCH</TT
> command
details.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The ECPG <TT
CLASS="COMMAND"
>DECLARE</TT
> command does not actually
cause a statement to be sent to the PostgreSQL backend. The
cursor is opened in the backend (using the
backend's <TT
CLASS="COMMAND"
>DECLARE</TT
> command) at the point when
the <TT
CLASS="COMMAND"
>OPEN</TT
> command is executed.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-TRANSACTIONS"
>33.3.3. Managing Transactions</A
></H2
><P
> In the default mode, statements are committed only when
<TT
CLASS="COMMAND"
>EXEC SQL COMMIT</TT
> is issued. The embedded SQL
interface also supports autocommit of transactions (similar to
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
> behavior) via the <TT
CLASS="OPTION"
>-t</TT
>
command-line option to <TT
CLASS="COMMAND"
>ecpg</TT
> (see <A
HREF="app-ecpg.html"
><SPAN
CLASS="APPLICATION"
>ecpg</SPAN
></A
>) or via the <TT
CLASS="LITERAL"
>EXEC SQL SET AUTOCOMMIT TO
ON</TT
> statement. In autocommit mode, each command is
automatically committed unless it is inside an explicit transaction
block. This mode can be explicitly turned off using <TT
CLASS="LITERAL"
>EXEC
SQL SET AUTOCOMMIT TO OFF</TT
>.
</P
><P
> The following transaction management commands are available:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>EXEC SQL COMMIT</TT
></DT
><DD
><P
> Commit an in-progress transaction.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL ROLLBACK</TT
></DT
><DD
><P
> Roll back an in-progress transaction.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>EXEC SQL SET AUTOCOMMIT TO ON</TT
></DT
><DD
><P
> Enable autocommit mode.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SET AUTOCOMMIT TO OFF</TT
></DT
><DD
><P
> Disable autocommit mode. This is the default.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-PREPARED"
>33.3.4. Prepared Statements</A
></H2
><P
> When the values to be passed to an SQL statement are not known at
compile time, or the same statement is going to be used many
times, then prepared statements can be useful.
</P
><P
> The statement is prepared using the
command <TT
CLASS="LITERAL"
>PREPARE</TT
>. For the values that are not
known yet, use the
placeholder <SPAN
CLASS="QUOTE"
>"<TT
CLASS="LITERAL"
>?</TT
>"</SPAN
>:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL PREPARE stmt1 FROM "SELECT oid, datname FROM pg_database WHERE oid = ?";</PRE
><P>
</P
><P
> If a statement returns a single row, the application can
call <TT
CLASS="LITERAL"
>EXECUTE</TT
> after
<TT
CLASS="LITERAL"
>PREPARE</TT
> to execute the statement, supplying the
actual values for the placeholders with a <TT
CLASS="LITERAL"
>USING</TT
>
clause:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL EXECUTE stmt1 INTO :dboid, :dbname USING 1;</PRE
><P>
</P
><P
> If a statement returns multiple rows, the application can use a
cursor declared based on the prepared statement. To bind input
parameters, the cursor must be opened with
a <TT
CLASS="LITERAL"
>USING</TT
> clause:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL PREPARE stmt1 FROM "SELECT oid,datname FROM pg_database WHERE oid > ?";
EXEC SQL DECLARE foo_bar CURSOR FOR stmt1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
EXEC SQL OPEN foo_bar USING 100;
...
while (1)
{
EXEC SQL FETCH NEXT FROM foo_bar INTO :dboid, :dbname;
...
}
EXEC SQL CLOSE foo_bar;</PRE
><P>
</P
><P
> When you don't need the prepared statement anymore, you should
deallocate it:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEALLOCATE PREPARE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
>;</PRE
><P>
</P
><P
> For more details about <TT
CLASS="LITERAL"
>PREPARE</TT
>,
see <A
HREF="ecpg-sql-prepare.html"
>PREPARE</A
>. Also
see <A
HREF="ecpg-dynamic.html"
>Section 33.5</A
> for more details about using
placeholders and input parameters.
</P
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