Getting Records Using the Cursor

Getting Records Using the Cursor
Prev	Chapter 4. Using Cursors	Next

Searching for Records
Working with Duplicate Records

To iterate over database records, from the first record to the last, simply open the cursor and then use the DBC->get() method. Note that you need to supply the DB_NEXT flag to this method. For example:

#include <db.h>
#include <string.h>

...

DB *my_database;
DBC *cursorp;
DBT key, data;
int ret;

/* Database open omitted for clarity */

/* Get a cursor */
my_database->cursor(my_database, NULL, &cursorp, 0); 

/* Initialize our DBTs. */
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
                                                                                                                               
/* Iterate over the database, retrieving each record in turn. */
while ((ret = cursorp->get(cursorp, &key, &data, DB_NEXT)) == 0) {
        /* Do interesting things with the DBTs here. */
}
if (ret != DB_NOTFOUND) {
        /* Error handling goes here */
}

/* Cursors must be closed */
if (cursorp != NULL) 
    cursorp->close(cursorp); 

if (my_database != NULL) 
    my_database->close(my_database, 0);

To iterate over the database from the last record to the first, use DB_PREV instead of DB_NEXT:

#include <db.h>
#include <string.h>

...

DB *my_database;
DBC *cursorp;
DBT key, data;
int ret;

/* Database open omitted for clarity */

/* Get a cursor */
my_database->cursor(my_database, NULL, &cursorp, 0); 

/* Initialize our DBTs. */
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
                                                                                                                               
/* Iterate over the database, retrieving each record in turn. */
while ((ret = cursorp->get(cursorp, &key,
      &data, DB_PREV)) == 0) {
        /* Do interesting things with the DBTs here. */
}
if (ret != DB_NOTFOUND) {
        /* Error handling goes here */
}

// Cursors must be closed
if (cursorp != NULL) 
    cursorp->close(cursorp); 

if (my_database != NULL)
    my_database->close(my_database, 0);

Searching for Records

You can use cursors to search for database records. You can search based on just a key, or you can search based on both the key and the data. You can also perform partial matches if your database supports sorted duplicate sets. In all cases, the key and data parameters of these methods are filled with the key and data values of the database record to which the cursor is positioned as a result of the search.

Also, if the search fails, then cursor's state is left unchanged and DB_NOTFOUND is returned.

To use a cursor to search for a record, use DBT->get(). When you use this method, you can provide the following flags:

Note

Notice in the following list that the cursor flags use the keyword SET when the cursor examines just the key portion of the records (in this case, the cursor is set to the record whose key matches the value provided to the cursor). Moreover, when the cursor uses the keyword GET, then the cursor is positioned to both the key and the data values provided to the cursor.

Regardless of the keyword you use to get a record with a cursor, the cursor's key and data DBTs are filled with the data retrieved from the record to which the cursor is positioned.

DB_SET

Moves the cursor to the first record in the database with the specified key.
DB_SET_RANGE

Identical to DB_SET unless you are using the BTree access. In this case, the cursor moves to the first record in the database whose key is greater than or equal to the specified key. This comparison is determined by the comparison function that you provide for the database. If no comparison function is provided, then the default lexicographical sorting is used.

For example, suppose you have database records that use the following strings as keys:
```
Alabama
Alaska
Arizona
```
Then providing a search key of Alaska moves the cursor to the second key noted above. Providing a key of Al moves the cursor to the first key (Alabama), providing a search key of Alas moves the cursor to the second key (Alaska), and providing a key of Ar moves the cursor to the last key (Arizona).
DB_GET_BOTH

Moves the cursor to the first record in the database that uses the specified key and data.

DB_GET_BOTH_RANGE

Moves the cursor to the first record in the database whose key matches the specified key and whose data is greater than or equal to the specified data. If the database supports duplicate records, then on matching the key, the cursor is moved to the duplicate record with the smallest data that is greater than or equal to the specified data.

For example, suppose your database uses BTree and it has database records that use the following key/data pairs:

Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence

then providing:

a search key of ...	and a search data of ...	moves the cursor to ...
Alaska	Fa	Alaska/Fairbanks
Arizona	Fl	Arizona/Florence
Alaska	An	Alaska/Anchorage

For example, assuming a database containing sorted duplicate records of U.S. States/U.S Cities key/data pairs (both as strings), then the following code fragment can be used to position the cursor to any record in the database and print its key/data values:

#include <db.h>
#include <string.h>

...

DBC *cursorp;
DBT key, data;
DB *dbp;
int ret;
char *search_data = "Fa";
char *search_key = "Alaska";

/* database open omitted for clarity */

/* Get a cursor */
dbp->cursor(dbp, NULL, &cursorp, 0);

/* Set up our DBTs */
key.data = search_key;
key.size = strlen(search_key) + 1;
data.data = search_data;
data.size = strlen(search_data) + 1;

/*
 * Position the cursor to the first record in the database whose
 * key matches the search key and whose data begins with the 
 * search data.
 */
ret = cursorp->get(cursorp, &key, &data, DB_GET_BOTH_RANGE);
if (!ret) {
    /* Do something with the data */
} else {
    /* Error handling goes here */
}

/* Close the cursor */
if (cursorp != NULL)
    cursorp->close(cursorp);

/* Close the database */
if (dbp != NULL)
    dbp->close(dbp, 0);

Working with Duplicate Records

A record is a duplicate of another record if the two records share the same key. For duplicate records, only the data portion of the record is unique.

Duplicate records are supported only for the BTree or Hash access methods. For information on configuring your database to use duplicate records, see Allowing Duplicate Records.

If your database supports duplicate records, then it can potentially contain multiple records that share the same key. By default, normal database get operations will only return the first such record in a set of duplicate records. Typically, subsequent duplicate records are accessed using a cursor. The following DBC->get() flags are interesting when working with databases that support duplicate records:

DB_NEXT, DB_PREV

Shows the next/previous record in the database, regardless of whether it is a duplicate of the current record. For an example of using these methods, see Getting Records Using the Cursor.
DB_GET_BOTH_RANGE

Useful for seeking the cursor to a specific record, regardless of whether it is a duplicate record. See Searching for Records for more information.
DB_NEXT_NODUP, DB_PREV_NODUP

Gets the next/previous non-duplicate record in the database. This allows you to skip over all the duplicates in a set of duplicate records. If you call DBC->get() with DB_PREV_NODUP, then the cursor is positioned to the last record for the previous key in the database. For example, if you have the following records in your database:
```
Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence
```
and your cursor is positioned to Alaska/Fairbanks, and you then call DBC->get() with DB_PREV_NODUP, then the cursor is positioned to Alabama/Florence. Similarly, if you call DBC->get() with DB_NEXT_NODUP, then the cursor is positioned to the first record corresponding to the next key in the database.

If there is no next/previous key in the database, then DB_NOTFOUND is returned, and the cursor is left unchanged.
DB_NEXT_DUP

Gets the next record that shares the current key. If the cursor is positioned at the last record in the duplicate set and you call DBC->get() with DB_NEXT_DUP, then DB_NOTFOUND is returned and the cursor is left unchanged.

For example, the following code fragment positions a cursor to a key and displays it and all its duplicates.

#include <db.h>
#include <string.h>

...

DB *dbp;
DBC *cursorp;
DBT key, data;
int ret;
char *search_key = "Al";

/* database open omitted for clarity */

/* Get a cursor */
dbp->cursor(dbp, NULL, &cursorp, 0);

/* Set up our DBTs */
key.data = search_key;
key.size = strlen(search_key) + 1;

/*
 * Position the cursor to the first record in the database whose
 * key and data begin with the correct strings.
 */
ret = cursorp->get(cursorp, &key, &data, DB_SET);
while (ret != DB_NOTFOUND) {
    printf("key: %s, data: %s\n", (char *)key.data, (char *)data.data);
    ret = cursorp->get(cursorp, &key, &data, DB_NEXT_DUP);
}

/* Close the cursor */
if (cursorp != NULL)
    cursorp->close(cursorp);

/* Close the database */
if (dbp != NULL)
    dbp->close(dbp, 0);