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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="joins"></a>Database Joins</h2>
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<div></div>
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<p>
If you have two or more secondary databases associated with a primary
database, then you can retrieve primary records based on the intersection of
multiple secondary entries. You do this using a
<span>join cursor.</span>
</p>
<p>
Throughout this document we have presented a
<span>structure</span>
that stores
information on grocery
<span>vendors.</span>
That
<span>structure</span>
is fairly simple with a limited
number of data members, few of which would be interesting from a query
perspective. But suppose, instead, that we were storing
information on something with many more characteristics that can be queried, such
as an automobile. In that case, you may be storing information such as
color, number of doors, fuel mileage, automobile type, number of
passengers, make, model, and year, to name just a few.
</p>
<p>
In this case, you would still likely be using some unique value to key your
primary entries (in the United States, the automobile's VIN would be
ideal for this purpose). You would then create a
<span>structure</span>
that identifies
all the characteristics of the automobiles in your inventory.
</p>
<p>
To query this data, you might then create multiple secondary databases,
one for each of the characteristics that you want to query. For
example, you might create a secondary for color, another for number of
doors, another for number of passengers, and so forth. Of course, you
will need a unique
<span>key extractor function</span>
for each such secondary database. You do
all of this using the concepts and techniques described throughout this
chapter.
</p>
<p>
Once you have created this primary database and all interesting
secondaries, what you have is the ability to retrieve automobile records
based on a single characteristic. You can, for example, find all the
automobiles that are red. Or you can find all the automobiles that have
four doors. Or all the automobiles that are minivans.
</p>
<p>
The next most natural step, then, is to form compound queries, or joins.
For example, you might want to find all the automobiles that are red,
and that were built by Toyota, and that are minivans. You can do this
using a
<span>join cursor.</span>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="joinUsage"></a>Using Join Cursors</h3>
</div>
</div>
<div></div>
</div>
<p>
To use a join cursor:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Open two or more
cursors
<span>for</span>
secondary databases that are associated with
the same primary database.
</p>
</li>
<li>
<p>
Position each such cursor to the secondary key
value in which you are interested. For example, to build on
the previous description, the cursor for the color
database is positioned to the <tt class="literal">red</tt> records
while the cursor for the model database is positioned to the
<tt class="literal">minivan</tt> records, and the cursor for the
make database is positioned to <tt class="literal">Toyota</tt>.
</p>
</li>
<li>
<p>
Create an array of cursors, and
place in it each of the cursors that are participating in your join query.
<span>Note that this array must be null terminated.</span>
</p>
</li>
<li>
<p>
Obtain a join cursor. You do this using the
<tt class="methodname">DB->join()</tt>
method. You must pass this method the array of secondary cursors that you
opened and positioned in the previous steps.
</p>
</li>
<li>
<p>
Iterate over the set of matching records
until
<span>the return code is not <tt class="literal">0</tt>.</span>
</p>
</li>
<li>
<p>
Close your cursor.
</p>
</li>
<li>
<p>
If you are done with them, close all your cursors.
</p>
</li>
</ul>
</div>
<p>
For example:
</p>
<a id="c_index9"></a>
<pre class="programlisting">#include <db.h>
#include <string.h>
...
DB *automotiveDB;
DB *automotiveColorDB;
DB *automotiveMakeDB;
DB *automotiveTypeDB;
DBC *color_curs, *make_curs, *type_curs, *join_curs;
DBC *carray[4];
DBT key, data;
int ret;
char *the_color = "red";
char *the_type = "minivan";
char *the_make = "Toyota";
/* Database and secondary database opens omitted for brevity.
* Assume a primary database handle:
* automotiveDB
* Assume 3 secondary database handles:
* automotiveColorDB -- secondary database based on automobile color
* automotiveMakeDB -- secondary database based on the manufacturer
* automotiveTypeDB -- secondary database based on automobile type
*/
/* initialize pointers and structures */
color_curs = NULL;
make_curs = NULL;
type_curs = NULL;
join_curs = NULL;
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
/* open the cursors */
if (( ret =
automotiveColorDB->cursor(automotiveColorDB, NULL,
&color_curs, 0)) != 0) {
/* Error handling goes here */
}
if (( ret =
automotiveMakeDB->cursor(automotiveMakeDB, NULL,
&make_curs, 0)) != 0) {
/* Error handling goes here */
}
if (( ret =
automotiveTypeDB->cursor(automotiveTypeDB, NULL,
&type_curs, 0)) != 0) {
/* Error handling goes here */
}
/* Position the cursors */
key.data = the_color;
key.size = strlen(the_color) + 1;
if ((ret = color_curs->get(color_curs, &key, &data, DB_SET)) != 0)
/* Error handling goes here */
key.data = the_make;
key.size = strlen(the_make) + 1;
if ((ret = make_curs->get(make_curs, &key, &data, DB_SET)) != 0)
/* Error handling goes here */
key.data = the_type;
key.size = strlen(the_type) + 1;
if ((ret = type_curs->get(type_curs, &key, &data, DB_SET)) != 0)
/* Error handling goes here */
/* Set up the cursor array */
carray[0] = color_curs;
carray[1] = make_curs;
carray[2] = type_curs;
carray[3] = NULL;
/* Create the join */
if ((ret = automotiveDB->join(automotiveDB, carray, &join_curs, 0)) != 0)
/* Error handling goes here */
/* Iterate using the join cursor */
while ((ret = join_curs->get(join_curs, &key, &data, 0)) == 0) {
/* Do interesting things with the key and data */
}
/*
* If we exited the loop because we ran out of records,
* then it has completed successfully.
*/
if (ret == DB_NOTFOUND) {
/*
* Close all our cursors and databases as is appropriate, and
* then exit with a normal exit status (0).
*/
} </pre>
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