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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="libgda-provider-parser"></a>SQL parser</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="sect1"><a href="libgda-provider-parser.html#id-1.9.10.3">Implementation overview</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="libgda-provider-parser.html#id-1.9.10.3.3">Generic SQL parser</a></span></dt>
<dt><span class="sect2"><a href="libgda-provider-parser.html#id-1.9.10.3.4">Provider specific SQL parser</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="ch47s02.html">Tips to write a custom parser</a></span></dt>
<dt>
<span class="refentrytitle"><a href="libgda-5.0-GdaSqlStatement.html">GdaSqlStatement</a></span><span class="refpurpose"> — SQL statement</span>
</dt>
</dl></div>
<p>
<span class="application">Libgda</span> implements a generic SQL parser which creates <a class="link" href="GdaStatement.html" title="GdaStatement">GdaStatement</a> objects from
an SQL string. If the database provider needs to implement its own parser because the generic one does not handle
the database specific SQL syntax, it can be done using instructions in this chapter. Otherwise, the provider's sources
can be cleared of any code related to the parser.
</p>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id-1.9.10.3"></a>Implementation overview</h2></div></div></div>
<p>
This section describes how the generic SQL parser and a provider specific parser are built regarding the files
and programs which are involved.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.9.10.3.3"></a>Generic SQL parser</h3></div></div></div>
<p>
The <a class="link" href="GdaSqlParser.html" title="GdaSqlParser">GdaSqlParser</a> object can parse any SQL string of any SQL dialect,
while always identifying the variables (which have a <span class="application">Libgda</span>'s specific syntax) in the string. If the parser
can identify a structure in the SQL string it understands, then it internally builds a
<a class="link" href="libgda-5.0-GdaSqlStatement.html#GdaSqlStatement" title="GdaSqlStatement">GdaSqlStatement</a> structure of the correct type, and if it cannot then is simply
delimits parts in the SQL string to identify variables and also builds a
<a class="link" href="libgda-5.0-GdaSqlStatement.html#GdaSqlStatement" title="GdaSqlStatement">GdaSqlStatement</a> structure but of
<a class="link" href="libgda-5.0-GdaSqlStatement.html#GDA-SQL-STATEMENT-UNKNOWN:CAPS">GDA_SQL_STATEMENT_UNKNOWN</a>. If the string
cannot be delimited and variables identified, then it returns an error (usually there is a quotes mismatch problem
within the SQL string).
</p>
<p>
Failing to identify a known structure in the SQL string can have several reasons:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>the SQL string is not one of the known types of statements (see
<a class="link" href="libgda-5.0-GdaSqlStatement.html#GdaSqlStatementType" title="enum GdaSqlStatementType">GdaSqlStatementType</a>)</p></li>
<li class="listitem"><p>the SQL uses some database specific extensions</p></li>
</ul></div>
<p>
</p>
<p>
The generic SQL parser implementation has its source files in the
<code class="filename">libgda/sql-parser</code> directory; the files which actually implement
the parser itself are the <code class="filename">parser.y</code>, <code class="filename">delimiter.y</code> and
<code class="filename">parser_tokens.h</code> files:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>The <code class="filename">parser.y</code> file contains the grammar used by the parser</p></li>
<li class="listitem"><p>The <code class="filename">delimiter.y</code> file contains the grammar used by the parser when it
is operating as a delimiter</p></li>
<li class="listitem"><p>The <code class="filename">parser_tokens.h</code> defines some hard coded tokens</p></li>
</ul></div>
<p>
</p>
<p>
The parser grammar files use the <a class="ulink" href="http://www.hwaci.com/sw/lemon/" target="_top">Lemon parser generator</a> syntax
which is a LALR parser similar to <span class="application">YACC</span> or <span class="application">bison</span>. The lexer part
however is not <span class="application">LEX</span> but is a custom one integrated in the
<code class="filename">gda-sql-parser.c</code> file (this allows a better integration between the lexer and parser parts).
</p>
<p>
The following figure illustrates the files involved and how they are produced and used to create
the generic SQL parser.
</p>
<div class="mediaobject"><img src="parser_gen.png" alt="Generic SQL parser's implementation"></div>
<p>
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>The white background indicate files which are sources
(part of <span class="application">Libgda</span>'s distribution)</p></li>
<li class="listitem"><p>The blue background indicate files that they are produced dynamically</p></li>
<li class="listitem">
<p>The pink background indicate programs that are compiled and used themselves in
the compilation process to generate files. These programs are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p><span class="application">lemon</span>: the lemon parser itself</p></li>
<li class="listitem"><p><span class="application">gen_def</span>: generated the "converters" arrays (see blow)</p></li>
</ul></div>
<p>
Note that none of these programs gets installed (and when cross compiling, they are compiled as programs
executing on the host actually making the compilation).
</p>
</li>
<li class="listitem"><p>The green background identifies components which are reused when implementing provider specific
parsers</p></li>
</ul></div>
<p>
</p>
<p>
The tokenizer (AKA lexer) generates values identified by the "L_" prefix (for example "L_BEGIN"). Because
the GdaSqlParser object uses the same lexer with at least two different parsers (namely the parser and delimiter
mentioned earlier), and because the Lemon parser generator generates its own value identifiers for tokens, there
is a conversion step (the "converter" block in the diagram) which converts the "L_" prefixed tokens with the ones
usable by each parser (both converters are defined as arrays in the <code class="filename">token_types.h</code> file.
</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.9.10.3.4"></a>Provider specific SQL parser</h3></div></div></div>
<p>
One wants to write a database specific SQL parser when:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>the SQL understood by the database differs from the generic SQL. For example
PostgreSQL associates priorities to the compound statement in a different way as the generic SQL.
In this case it is strongly recommended to write a custom SQL parser</p></li>
<li class="listitem"><p>the SQL understood by the database has specific extensions</p></li>
</ul></div>
<p>
</p>
<p>
Using the same background color conventions as the previous diagram, the following diagram illustrates
the files involved and how they are produced and used to create a provider specific SQL parser:
</p>
<p>
</p>
<div class="mediaobject"><img src="parser_prov.png" alt="Provider specific SQL parser's implementation"></div>
<p>
</p>
<p>
The key differences are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>The delimiter part of the <a class="link" href="GdaSqlParser.html" title="GdaSqlParser">GdaSqlParser</a> object
is the same as for the generic SQL parser implementation</p></li>
<li class="listitem"><p>While the <span class="application">lemon</span> program is the same as for the generic SQL parser,
the <span class="application">gen_def</span> is different, and takes as its input the ".h" file produced by
the <span class="application">lemon</span> program and the <code class="filename">libgda/sql-parser/token_types.h</code>.
</p></li>
</ul></div>
<p>
</p>
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