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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-CONNECT"
>33.2. Managing Database Connections</A
></H1
><P
> This section describes how to open, close, and switch database
connections.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-CONNECTING"
>33.2.1. Connecting to the Database Server</A
></H2
><P
> One connects to a database using the following statement:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL CONNECT TO <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>AS <TT
CLASS="REPLACEABLE"
><I
>connection-name</I
></TT
></SPAN
>] [<SPAN
CLASS="OPTIONAL"
>USER <TT
CLASS="REPLACEABLE"
><I
>user-name</I
></TT
></SPAN
>];</PRE
><P>
The <TT
CLASS="REPLACEABLE"
><I
>target</I
></TT
> can be specified in the
following ways:
<P
></P
></P><UL
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
>[<SPAN
CLASS="OPTIONAL"
>@<TT
CLASS="REPLACEABLE"
><I
>hostname</I
></TT
></SPAN
>][<SPAN
CLASS="OPTIONAL"
>:<TT
CLASS="REPLACEABLE"
><I
>port</I
></TT
></SPAN
>]</TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>tcp:postgresql://<TT
CLASS="REPLACEABLE"
><I
>hostname</I
></TT
>[<SPAN
CLASS="OPTIONAL"
>:<TT
CLASS="REPLACEABLE"
><I
>port</I
></TT
></SPAN
>][<SPAN
CLASS="OPTIONAL"
>/<TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></SPAN
>][<SPAN
CLASS="OPTIONAL"
>?<TT
CLASS="REPLACEABLE"
><I
>options</I
></TT
></SPAN
>]</TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>unix:postgresql://<TT
CLASS="REPLACEABLE"
><I
>hostname</I
></TT
>[<SPAN
CLASS="OPTIONAL"
>:<TT
CLASS="REPLACEABLE"
><I
>port</I
></TT
></SPAN
>][<SPAN
CLASS="OPTIONAL"
>/<TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></SPAN
>][<SPAN
CLASS="OPTIONAL"
>?<TT
CLASS="REPLACEABLE"
><I
>options</I
></TT
></SPAN
>]</TT
>
</P
></LI
><LI
><P
> an SQL string literal containing one of the above forms
</P
></LI
><LI
><P
> a reference to a character variable containing one of the above forms (see examples)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>DEFAULT</TT
>
</P
></LI
></UL
><P>
If you specify the connection target literally (that is, not
through a variable reference) and you don't quote the value, then
the case-insensitivity rules of normal SQL are applied. In that
case you can also double-quote the individual parameters separately
as needed. In practice, it is probably less error-prone to use a
(single-quoted) string literal or a variable reference. The
connection target <TT
CLASS="LITERAL"
>DEFAULT</TT
> initiates a connection
to the default database under the default user name. No separate
user name or connection name can be specified in that case.
</P
><P
> There are also different ways to specify the user name:
<P
></P
></P><UL
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>username</I
></TT
></TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>username</I
></TT
>/<TT
CLASS="REPLACEABLE"
><I
>password</I
></TT
></TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>username</I
></TT
> IDENTIFIED BY <TT
CLASS="REPLACEABLE"
><I
>password</I
></TT
></TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>username</I
></TT
> USING <TT
CLASS="REPLACEABLE"
><I
>password</I
></TT
></TT
>
</P
></LI
></UL
><P>
As above, the parameters <TT
CLASS="REPLACEABLE"
><I
>username</I
></TT
> and
<TT
CLASS="REPLACEABLE"
><I
>password</I
></TT
> can be an SQL identifier, an
SQL string literal, or a reference to a character variable.
</P
><P
> The <TT
CLASS="REPLACEABLE"
><I
>connection-name</I
></TT
> is used to handle
multiple connections in one program. It can be omitted if a
program uses only one connection. The most recently opened
connection becomes the current connection, which is used by default
when an SQL statement is to be executed (see later in this
chapter).
</P
><P
> Here are some examples of <TT
CLASS="COMMAND"
>CONNECT</TT
> statements:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL CONNECT TO mydb@sql.mydomain.com;
EXEC SQL CONNECT TO unix:postgresql://sql.mydomain.com/mydb AS myconnection USER john;
EXEC SQL BEGIN DECLARE SECTION;
const char *target = "mydb@sql.mydomain.com";
const char *user = "john";
const char *passwd = "secret";
EXEC SQL END DECLARE SECTION;
...
EXEC SQL CONNECT TO :target USER :user USING :passwd;
/* or EXEC SQL CONNECT TO :target USER :user/:passwd; */</PRE
><P>
The last form makes use of the variant referred to above as
character variable reference. You will see in later sections how C
variables can be used in SQL statements when you prefix them with a
colon.
</P
><P
> Be advised that the format of the connection target is not
specified in the SQL standard. So if you want to develop portable
applications, you might want to use something based on the last
example above to encapsulate the connection target string
somewhere.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-SET-CONNECTION"
>33.2.2. Choosing a Connection</A
></H2
><P
> SQL statements in embedded SQL programs are by default executed on
the current connection, that is, the most recently opened one. If
an application needs to manage multiple connections, then there are
two ways to handle this.
</P
><P
> The first option is to explicitly choose a connection for each SQL
statement, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL AT <TT
CLASS="REPLACEABLE"
><I
>connection-name</I
></TT
> SELECT ...;</PRE
><P>
This option is particularly suitable if the application needs to
use several connections in mixed order.
</P
><P
> If your application uses multiple threads of execution, they cannot share a
connection concurrently. You must either explicitly control access to the connection
(using mutexes) or use a connection for each thread.
</P
><P
> The second option is to execute a statement to switch the current
connection. That statement is:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL SET CONNECTION <TT
CLASS="REPLACEABLE"
><I
>connection-name</I
></TT
>;</PRE
><P>
This option is particularly convenient if many statements are to be
executed on the same connection.
</P
><P
> Here is an example program managing multiple database connections:
</P><PRE
CLASS="PROGRAMLISTING"
>#include <stdio.h>
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
int
main()
{
EXEC SQL CONNECT TO testdb1 AS con1 USER testuser;
EXEC SQL CONNECT TO testdb2 AS con2 USER testuser;
EXEC SQL CONNECT TO testdb3 AS con3 USER testuser;
/* This query would be executed in the last opened database "testdb3". */
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb3)\n", dbname);
/* Using "AT" to run a query in "testdb2" */
EXEC SQL AT con2 SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb2)\n", dbname);
/* Switch the current connection to "testdb1". */
EXEC SQL SET CONNECTION con1;
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb1)\n", dbname);
EXEC SQL DISCONNECT ALL;
return 0;
}</PRE
><P>
This example would produce this output:
</P><PRE
CLASS="SCREEN"
>current=testdb3 (should be testdb3)
current=testdb2 (should be testdb2)
current=testdb1 (should be testdb1)</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-DISCONNECT"
>33.2.3. Closing a Connection</A
></H2
><P
> To close a connection, use the following statement:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DISCONNECT [<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>connection</I
></TT
></SPAN
>];</PRE
><P>
The <TT
CLASS="REPLACEABLE"
><I
>connection</I
></TT
> can be specified
in the following ways:
<P
></P
></P><UL
><LI
><P
> <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>connection-name</I
></TT
></TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>DEFAULT</TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>CURRENT</TT
>
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>ALL</TT
>
</P
></LI
></UL
><P>
If no connection name is specified, the current connection is
closed.
</P
><P
> It is good style that an application always explicitly disconnect
from every connection it opened.
</P
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