gsg/C/Positioning.html

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          <th colspan="3" align="center">Getting Records Using the Cursor</th>
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          <td width="20%" align="left"><a accesskey="p" href="Cursors.html">Prev</a> </td>
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="Positioning"></a>Getting Records Using the Cursor</h2>
          </div>
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      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="Positioning.html#cursorsearch">Searching for Records</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="Positioning.html#getdups">Working with Duplicate Records</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        To iterate over database records, from the first record to
        the last, simply open the cursor and then use the

            <code class="methodname">DBC-&gt;get()</code>

        method.
        <span>Note that you need to supply the
        <code class="literal">DB_NEXT</code> flag to this method.</span>
        For example:
     </p>
      <a id="c_cursor3"></a>
      <pre class="programlisting">#include &lt;db.h&gt;
#include &lt;string.h&gt;

...

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

/* Database open omitted for clarity */

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

/* Initialize our DBTs. */
memset(&amp;key, 0, sizeof(DBT));
memset(&amp;data, 0, sizeof(DBT));

/* Iterate over the database, retrieving each record in turn. */
while ((ret = cursorp-&gt;get(cursorp, &amp;key, &amp;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-&gt;close(cursorp);

if (my_database != NULL)
    my_database-&gt;close(my_database, 0);</pre>
      <p>
        To iterate over the database from the last record to the first, use
        <code class="literal">DB_PREV</code> instead of <code class="literal">DB_NEXT</code>:
    </p>
      <a id="c_cursor4"></a>
      <pre class="programlisting">#include &lt;db.h&gt;
#include &lt;string.h&gt;

...

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

/* Database open omitted for clarity */

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

/* Initialize our DBTs. */
memset(&amp;key, 0, sizeof(DBT));
memset(&amp;data, 0, sizeof(DBT));

/* Iterate over the database, retrieving each record in turn. */
while ((ret = cursorp-&gt;get(cursorp, &amp;key,
      &amp;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-&gt;close(cursorp);

if (my_database != NULL)
    my_database-&gt;close(my_database, 0);</pre>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="cursorsearch"></a>Searching for Records</h3>
            </div>
          </div>
        </div>
        <p>
        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.
      </p>
        <p>
        Also, if the search fails, then cursor's state is left unchanged
        and

            <code class="literal">DB_NOTFOUND</code>
        is returned.


      </p>
        <p>
        To use a cursor to search for a record, use
            <span>DBT-&gt;get()<code class="methodname"></code>.</span>

        When you use this method, you can provide the following flags:
    </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
            Notice in the following list that the cursor flags use the
            keyword <code class="literal">SET</code> 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 <code class="literal">GET</code>,
            then the cursor is positioned to both the key
            <span class="emphasis"><em>and</em></span> the data values provided to the cursor.
        </p>
          <p>
            Regardless of the keyword you use to get a record with a cursor, the
            cursor's key and data
                <span><code class="classname">DBT</code>s</span>

            are filled with the data retrieved from the record to which the
            cursor is positioned.
        </p>
        </div>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>

            <code class="literal">DB_SET</code>
           </p>
              <p>
            Moves the cursor to the first record in the database with
            the specified key.
          </p>
            </li>
            <li>
              <p>

            <code class="literal">DB_SET_RANGE</code>
          </p>
              <p>
            <span>Identical to
                <code class="literal">DB_SET</code>

            unless you are using the BTree access. In this case, the cursor
            moves</span>


            to the first record in the database whose
            key is greater than or equal to the specified key. This comparison
            is determined by the

                <span>comparison function</span>
            that you provide for the database. If no

                <span>comparison function</span>
                is provided, then the default

                lexicographical sorting is used.
          </p>
              <p>
            For example, suppose you have database records that use the
            following

                <span>strings</span>
            as keys:
          </p>
              <pre class="programlisting">Alabama
Alaska
Arizona</pre>
              <p>
            Then providing a search key of <code class="literal">Alaska</code> moves the
            cursor to the second key noted above. Providing a key of
            <code class="literal">Al</code> moves the cursor to the first key (<code class="literal">Alabama</code>), providing
            a search key of <code class="literal">Alas</code> moves the cursor to the second key
            (<code class="literal">Alaska</code>), and providing a key of <code class="literal">Ar</code> moves the
            cursor to the last key (<code class="literal">Arizona</code>).
          </p>
            </li>
            <li>
              <p>

            <code class="literal">DB_GET_BOTH</code>
           </p>
              <p>
            Moves the cursor to the first record in the database that uses
            the specified key and data.
          </p>
            </li>
            <li>
              <p>

            <code class="literal">DB_GET_BOTH_RANGE</code>
          </p>
              <p>
            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.
          </p>
              <p>
            For example,

                <span>suppose your database uses BTree
                and it has </span>
            database records that use the following key/data pairs:
          </p>
              <pre class="programlisting">Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence </pre>
              <p>then providing:</p>
              <div class="informaltable">
                <table border="1" width="80%">
                  <colgroup>
                    <col />
                    <col />
                    <col />
                  </colgroup>
                  <thead>
                    <tr>
                      <th>a search key of ...</th>
                      <th>and a search data of ...</th>
                      <th>moves the cursor to ...</th>
                    </tr>
                  </thead>
                  <tbody>
                    <tr>
                      <td>Alaska</td>
                      <td>Fa</td>
                      <td>Alaska/Fairbanks</td>
                    </tr>
                    <tr>
                      <td>Arizona</td>
                      <td>Fl</td>
                      <td>Arizona/Florence</td>
                    </tr>
                    <tr>
                      <td>Alaska</td>
                      <td>An</td>
                      <td>Alaska/Anchorage</td>
                    </tr>
                  </tbody>
                </table>
              </div>
            </li>
          </ul>
        </div>
        <p>
        For example, assuming a database containing sorted duplicate records of
        U.S. States/U.S Cities key/data pairs (both as

            <span>strings),</span>
        then the following code fragment can be used to position the cursor
        to any record in the database and print its key/data values:

      </p>
        <a id="c_cursor5"></a>
        <pre class="programlisting">#include &lt;db.h&gt;
#include &lt;string.h&gt;

...

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-&gt;cursor(dbp, NULL, &amp;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-&gt;get(cursorp, &amp;key, &amp;data, DB_GET_BOTH_RANGE);
if (!ret) {
    /* Do something with the data */
} else {
    /* Error handling goes here */
}

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

/* Close the database */
if (dbp != NULL)
    dbp-&gt;close(dbp, 0); </pre>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="getdups"></a>Working with Duplicate Records</h3>
            </div>
          </div>
        </div>
        <p>
        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.
      </p>
        <p>
        Duplicate records are supported only for the BTree or Hash access methods.
        For information on configuring your database to use duplicate records,
        see <a class="xref" href="btree.html#duplicateRecords" title="Allowing Duplicate Records">Allowing Duplicate Records</a>.
      </p>
        <p>
		If your database supports duplicate records, then it can potentially
		contain multiple records that share the same key.


        <span>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.
        </span>

        The following

            <span><code class="methodname">DBC-&gt;get()</code> flags</span>

        are interesting when working with databases that support duplicate records:
	  </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>

            <span>
                <code class="literal">DB_NEXT</code>,
                <code class="literal">DB_PREV</code>
            </span>
          </p>
              <p>
            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 <a class="xref" href="Positioning.html" title="Getting Records Using the Cursor">Getting Records Using the Cursor</a>.
          </p>
            </li>
            <li>
              <p>

            <code class="literal">DB_GET_BOTH_RANGE</code>
          </p>
              <p>
            Useful for seeking the cursor to a specific record, regardless of
            whether it is a duplicate record. See <a class="xref" href="Positioning.html#cursorsearch" title="Searching for Records">Searching for Records</a> for more
            information.
          </p>
            </li>
            <li>
              <p>

            <span>
                <code class="literal">DB_NEXT_NODUP</code>,
                <code class="literal">DB_PREV_NODUP</code>
            </span>
          </p>
              <p>
            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

                <span>
                    <code class="methodname">DBC-&gt;get()</code>

                    with <code class="literal">DB_PREV_NODUP</code>,
                </span>
            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:
          </p>
              <pre class="programlisting">Alabama/Athens
Alabama/Florence
Alaska/Anchorage
Alaska/Fairbanks
Arizona/Avondale
Arizona/Florence</pre>
              <p>
          and your cursor is positioned to <code class="literal">Alaska/Fairbanks</code>,
          and you then call

                <span>
                        <code class="methodname">DBC-&gt;get()</code>

                    with <code class="literal">DB_PREV_NODUP</code>,
                </span>
          then the cursor is positioned to Alabama/Florence. Similarly, if
          you call

                <span>
                    <code class="methodname">DBC-&gt;get()</code>

                    with <code class="literal">DB_NEXT_NODUP</code>,
                </span>

          then the cursor is positioned to the first record corresponding to
          the next key in the database.
          </p>
              <p>
            If there is no next/previous key in the database, then

                <code class="literal">DB_NOTFOUND</code>
            is returned, and the cursor is left unchanged.
          </p>
            </li>
            <li>
              <p>

                <code class="literal">DB_NEXT_DUP</code>
          </p>
              <p>

            Gets the

                <span>next</span>
            record that shares the current key. If the
            cursor is positioned at the last record in the duplicate set and
            you call

                <span>
                    <code class="methodname">DBC-&gt;get()</code>

                   with <code class="literal">DB_NEXT_DUP</code>,
                </span>

            then

                <code class="literal">DB_NOTFOUND</code>
            is returned and the cursor is left unchanged.

          </p>
            </li>
          </ul>
        </div>
        <p>
        For example, the following code fragment positions a cursor to a key


        <span>and displays it and all its
        duplicates.</span>


      </p>
        <a id="c_cursor6"></a>
        <pre class="programlisting">#include &lt;db.h&gt;
#include &lt;string.h&gt;

...

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

/* database open omitted for clarity */

/* Get a cursor */
dbp-&gt;cursor(dbp, NULL, &amp;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-&gt;get(cursorp, &amp;key, &amp;data, DB_SET);
while (ret != DB_NOTFOUND) {
    printf("key: %s, data: %s\n", (char *)key.data, (char *)data.data);
    ret = cursorp-&gt;get(cursorp, &amp;key, &amp;data, DB_NEXT_DUP);
}

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

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