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>Chapter 33. <SPAN
CLASS="APPLICATION"
>ECPG</SPAN
> - Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C</TD
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><A
NAME="ECPG-CONCEPT"
>33.1. The Concept</A
></H1
><P
> An embedded SQL program consists of code written in an ordinary
programming language, in this case C, mixed with SQL commands in
specially marked sections. To build the program, the source code (<TT
CLASS="FILENAME"
>*.pgc</TT
>)
is first passed through the embedded SQL preprocessor, which converts it
to an ordinary C program (<TT
CLASS="FILENAME"
>*.c</TT
>), and afterwards it can be processed by a C
compiler. (For details about the compiling and linking see <A
HREF="ecpg-process.html"
>Section 33.10</A
>).
Converted ECPG applications call functions in the libpq library
through the embedded SQL library (ecpglib), and communicate with
the PostgreSQL server using the normal frontend-backend protocol.
</P
><P
> Embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> has advantages over other methods
for handling <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> commands from C code. First, it
takes care of the tedious passing of information to and from
variables in your <ACRONYM
CLASS="ACRONYM"
>C</ACRONYM
> program. Second, the SQL
code in the program is checked at build time for syntactical
correctness. Third, embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> in C is
specified in the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> standard and supported by
many other <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> database systems. The
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> implementation is designed to match this
standard as much as possible, and it is usually possible to port
embedded <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> programs written for other SQL
databases to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> with relative
ease.
</P
><P
> As already stated, programs written for the embedded
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> interface are normal C programs with special
code inserted to perform database-related actions. This special
code always has the form:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL ...;</PRE
><P>
These statements syntactically take the place of a C statement.
Depending on the particular statement, they can appear at the
global level or within a function. Embedded
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> statements follow the case-sensitivity rules
of normal <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> code, and not those of C.
</P
><P
> The following sections explain all the embedded SQL statements.
</P
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