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 join cursor.

Throughout this document we have presented a structure that stores information on grocery vendors. That structure 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 queryable characteristics, 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.

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 structure that identifies all the characteristics of the automobiles in your inventory.

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 key extractor function for each such secondary database. You do all of this using the concepts and techniques described throughout this chapter.

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.

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 join cursor.

#include <db_cxx.h>
#include <string.h>

...

// Exception handling omitted

int ret;

Db automotiveDB(NULL, 0);
Db automotiveColorDB(NULL, 0);
Db automotiveMakeDB(NULL, 0);
Db automotiveTypeDB(NULL, 0);

// Database and secondary database opens omitted for brevity.
// Assume a primary database:
//   automotiveDB
// Assume 3 secondary databases:
//   automotiveColorDB  -- secondary database based on automobile color
//   automotiveMakeDB  -- secondary database based on the manufacturer
//   automotiveTypeDB  -- secondary database based on automobile type

// Position the cursors
Dbc *color_curs;
automotiveColorDB.cursor(NULL, &color_curs, 0);
char *the_color = "red";
Dbt key(the_color, strlen(the_color) + 1);
Dbt data;
if ((ret = color_curs->get(&key, &data, DB_SET)) != 0) {
    // Error handling goes here
}

Dbc *make_curs;
automotiveMakeDB.cursor(NULL, &make_curs, 0);
char *the_make = "Toyota";
key.set_data(the_make);
key.set_size(strlen(the_make) + 1);
if ((ret = make_curs->get(&key, &data, DB_SET)) != 0) {
    // Error handling goes here
}

Dbc *type_curs; 
automotiveTypeDB.cursor(NULL, &type_curs, 0);
char *the_type = "minivan";
key.set_data(the_type);
key.set_size(strlen(the_type) + 1);
if ((ret = type_curs->get(&key, &data, DB_SET)) != 0) {
    // Error handling goes here
}

// Set up the cursor array
Dbc *carray[4];
carray[0] = color_curs;
carray[1] = make_curs;
carray[2] = type_curs;
carray[3] = NULL;

// Create the join
Dbc *join_curs;
if ((ret = automotiveDB.join(carray, &join_curs, 0)) != 0) {
    // Error handling goes here
}

// Iterate using the join cursor
while ((ret = join_curs->get(&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). 
}