dbviewreaddata.htm

The goal of this library is to make ODBC recordsets look just like an STL container. As a user, you

<h3>Accessing a Table in Four Easy Steps:</h3>

<p><font face="Times Roman"><br>
<strong>1. Define an object to hold the rows from your query.</strong></font></p>

<p><font face="Times Roman"><strong>2. Define an association
between fields in your query and fields in your object. This is
what we call a 'BCA', which is short for Bind Column Addresses.
In the example below, this is done via the functor &quot;BCAExample&quot;.
The job of the BCA is to equate SQL fields with object fields via
the '==' operator which will then establish ODBC bindings to move
data to or from a user query.</strong></font></p>

<p><font face="Times Roman"><strong>3. Create a view to select
records from. This view is built from the template DBView and
establishes which table(s) you want to access, what fields you
want to look at (via the BCA), and an optional where clause to
further limit the set of records that you are working with. The
DBView template forms a semi-Container in the STL sense.</strong></font><a
href="#FNT0"><font size="2" face="Times Roman"><sup><strong>1</strong></sup></font></a><font
face="Times Roman"><strong>.</strong></font></p>

<p><font face="Times Roman"><strong>4. Use the DBView container
to obtain an iterator to SELECT, INSERT, UPDATE or DELETE records
from your view. These iterators may be used to either populate
STL containers or apply algorithms from the Standard Template
library.<br>
</strong></font></p>

<pre><code>
In all the examples that follow we will assume that our database contains a table called DB_EXAMPLE of the form

SQL&gt; desc db_example;
Name                            Type
------------------------------- --------
INT_VALUE                       INTEGER
STRING_VALUE                    VARCHAR
DOUBLE_VALUE                    FLOAT
EXAMPLE_LONG                    INTEGER
EXAMPLE_DATE                    DATE

<span class="codeComment">// STEP 1 ////
// &quot;Example&quot; class to hold rows from our database table</span>
class Example
{
  public:                                <span class="codeComment">// tablename.columnname:</span>
	int exampleInt;                 <span class="codeComment">// DB_EXAMPLE.INT_VALUE</span>
	string exampleStr;              <span class="codeComment">// DB_EXAMPLE.STRING_VALUE</span>
	double exampleDouble;           <span class="codeComment">// DB_EXAMPLE.DOUBLE_VALUE</span>
	long exampleLong;               <span class="codeComment">// DB_EXAMPLE.EXAMPLE_LONG</span>
	TIMESTAMP_STRUCT exampleDate;   <span class="codeComment">// DB_EXAMPLE.EXAMPLE_DATE</span>

	Example(int exInt, const string &amp;exStr, double exDouble, long exLong,
		const TIMESTAMP_STRUCT &amp;exDate) :
	   exampleInt(exInt), exampleStr(exStr), exampleDouble(exDouble), exampleLong(exLong),
	   exampleDate(exDate)
	{ }

};

<span class="codeComment">// STEP 2 ////</span>
<span class="codeComment">// Create an association between table columns and fields in our object</span>
template&lt;&gt; class dtl::DefaultBCA&lt;Example&gt;
{
public:
	void operator()(BoundIOs &amp;cols, Example &amp;rowbuf)
    	{
	   cols[&quot;INT_VALUE&quot;] == rowbuf.exampleInt;
	   cols[&quot;STRING_VALUE&quot;] == rowbuf.exampleStr;
	   cols[&quot;DOUBLE_VALUE&quot;] == rowbuf.exampleDouble;
	   cols[&quot;EXAMPLE_LONG&quot;] == rowbuf.exampleLong;
	   cols[&quot;EXAMPLE_DATE&quot;] == rowbuf.exampleDate;
	}
}

<span class="codeComment">// STEP 3 &amp; 4
// Read the contents of the DB_EXAMPLE table and return a vector of the
// resulting rows</span>
vector&lt;Example&gt; ReadData() {
	// Read the data
	vector&lt;Example&gt; results;
	DBView&lt;Example&gt; view(&quot;DB_EXAMPLE&quot;);

	DBView&lt;Example&gt;::select_iterator read_it = view.begin();
	for ( ; read_it != view.end();  read_it++)
	{
		results.push_back(*read_it);
	}
	return results;
}
</code></pre>

<p><a name="FNT0"></a></p>

<p><font face="Times Roman">1</font><font face="Arial"> </font><font
face="Times Roman">See </font><a
href="http://www.sgi.com/tech/stl/Container.html"><font
color="#0000FF" face="Times Roman"><u>http://www.sgi.com/tech/stl/Container.html</u></font></a><font
color="#0000FF" face="Times Roman"> </font><font
face="Times Roman">for the definition of an STL container, we
call DBView a <b>semi</b> container because it supports all
standard container methods <b>except</b> size(), max_size() and
empty(). We explain why these were left out by design in the
documentation for the DBView template. <br>
</font></p>