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|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Cursor operations</title>
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<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Cursor operations</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="am_foreign.html">Prev</a> </td>
<th width="60%" align="center">Chapter 3. Access Method Operations </th>
<td width="20%" align="right"> <a accesskey="n" href="am_misc.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="am_cursor"></a>Cursor operations</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_curget">Retrieving records with a
cursor</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_curput">Storing records with a cursor</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_curdel">Deleting records with a cursor</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_curdup">Duplicating a cursor</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_join">Equality Join</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_count">Data item count</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="am_cursor.html#am_curclose">Cursor close</a>
</span>
</dt>
</dl>
</div>
<p>
A database cursor refers to a single key/data pair in the
database. It supports traversal of the database and is the
only way to access individual duplicate data items. Cursors
are used for operating on collections of records, for
iterating over a database, and for saving handles to
individual records, so that they can be modified after they
have been read.
</p>
<p>
The <a href="../api_reference/C/dbcursor.html" class="olink">DB->cursor()</a> method opens a cursor into a database. Upon
return the cursor is uninitialized, cursor positioning occurs
as part of the first cursor operation.
</p>
<p>
Once a database cursor has been opened, records may be
retrieved (<a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>), stored (<a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a>), and deleted
(<a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a>).
</p>
<p>
Additional operations supported by the cursor handle
include duplication (<a href="../api_reference/C/dbcdup.html" class="olink">DBC->dup()</a>), equality join (<a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a>), and
a count of duplicate data items (<a href="../api_reference/C/dbccount.html" class="olink">DBC->count()</a>). Cursors are
eventually closed using <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a>.
</p>
<p>
For more information on the operations supported by the
cursor handle, see the <a href="../api_reference/C/dbc.html#dbclist" class="olink">Database Cursors and Related
Methods</a> section in the
<em class="citetitle">Berkeley DB C API Reference Guide.</em>
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_curget"></a>Retrieving records with a
cursor</h3>
</div>
</div>
</div>
<p>
The <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> method retrieves records from the database
using a cursor. The <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> method takes a flag which
controls how the cursor is positioned within the database and
returns the key/data item associated with that positioning.
Similar to <a href="../api_reference/C/dbget.html" class="olink">DB->get()</a>, <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> may also take a supplied key and
retrieve the data associated with that key from the database.
There are several flags that you can set to customize
retrieval.
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp413624"></a>Cursor position flags</h4>
</div>
</div>
</div>
<div class="variablelist">
<dl>
<dt>
<span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_FIRST" class="olink">DB_FIRST</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_LAST" class="olink">DB_LAST</a></span>
</dt>
<dd>
Return the first (last) record in the
database.
</dd>
<dt>
<span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT" class="olink">DB_NEXT</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_PREV" class="olink">DB_PREV</a></span>
</dt>
<dd>
Return the next (previous) record in the
database.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT_DUP" class="olink">DB_NEXT_DUP</a>
</span>
</dt>
<dd>
Return the next record in the database, if
it is a duplicate data item for the current key.
For Heap databases, this flag always results in
the cursor returning the
<code class="literal">DB_NOTFOUND</code> error.
</dd>
<dt>
<span class="term"><a href="../api_reference/C/dbcget.html#dbcget_DB_NEXT_NODUP" class="olink">DB_NEXT_NODUP</a>, <a href="../api_reference/C/dbcget.html#dbcget_DB_PREV_NODUP" class="olink">DB_PREV_NODUP</a></span>
</dt>
<dd>
Return the next (previous) record in the
database that is not a duplicate data item for the
current key.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a>
</span>
</dt>
<dd>
Return the record from the database to
which the cursor currently refers.
</dd>
</dl>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp419144"></a>Retrieving specific key/data pairs</h4>
</div>
</div>
</div>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_SET" class="olink">DB_SET</a>
</span>
</dt>
<dd>
Return the record from the database that
matches the supplied key. In the case of
duplicates the first duplicate is returned and the
cursor is positioned at the beginning of the
duplicate list. The user can then traverse the
duplicate entries for the key.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_SET_RANGE" class="olink">DB_SET_RANGE</a>
</span>
</dt>
<dd>
Return the smallest record in the database
greater than or equal to the supplied key. This
functionality permits partial key matches and
range searches in the Btree access method.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH" class="olink">DB_GET_BOTH</a>
</span>
</dt>
<dd>
Return the record from the database that
matches both the supplied key and data items. This
is particularly useful when there are large
numbers of duplicate records for a key, as it
allows the cursor to easily be positioned at the
correct place for traversal of some part of a
large set of duplicate records.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH_RANGE" class="olink">DB_GET_BOTH_RANGE</a>
</span>
</dt>
<dd>
If used on a database configured for sorted
duplicates, this returns the smallest record in
the database greater than or equal to the supplied
key and data items. If used on a database that is
<span class="emphasis"><em>not</em></span> configured for sorted
duplicates, this flag behaves identically to
<code class="literal">DB_GET_BOTH</code>.
</dd>
</dl>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp409264"></a>Retrieving based on record numbers</h4>
</div>
</div>
</div>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_SET_RECNO" class="olink">DB_SET_RECNO</a>
</span>
</dt>
<dd>
If the underlying database is a Btree, and
was configured so that it is possible to search it
by logical record number, retrieve a specific
record based on a record number argument.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_GET_RECNO" class="olink">DB_GET_RECNO</a>
</span>
</dt>
<dd>
If the underlying database is a Btree, and
was configured so that it is possible to search it
by logical record number, return the record number
for the record to which the cursor refers.
</dd>
</dl>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp418264"></a>Special-purpose flags</h4>
</div>
</div>
</div>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/dbget.html#dbget_DB_CONSUME" class="olink">DB_CONSUME</a>
</span>
</dt>
<dd>
Read-and-delete: the first record (the
head) of the queue is returned and deleted. The
underlying database must be a Queue.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_RMW" class="olink">DB_RMW</a>
</span>
</dt>
<dd>
Read-modify-write: acquire write locks
instead of read locks during retrieval. This can
enhance performance in threaded applications by
reducing the chance of deadlock.
</dd>
</dl>
</div>
<p>
In all cases, the cursor is repositioned by a <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>
operation to point to the newly-returned key/data pair in
the database.
</p>
<p>
The following is a code example showing a cursor
walking through a database and displaying the records it
contains to the standard output:
</p>
<a id="prog_am19"></a>
<pre class="programlisting">int
display(char *database)
{
DB *dbp;
DBC *dbcp;
DBT key, data;
int close_db, close_dbc, ret;
close_db = close_dbc = 0;
/* Open the database. */
if ((ret = db_create(&dbp, NULL, 0)) != 0) {
fprintf(stderr,
"%s: db_create: %s\n", progname, db_strerror(ret));
return (1);
}
close_db = 1;
/* Turn on additional error output. */
dbp->set_errfile(dbp, stderr);
dbp->set_errpfx(dbp, progname);
/* Open the database. */
if ((ret = dbp->open(dbp, NULL, database, NULL,
DB_UNKNOWN, DB_RDONLY, 0)) != 0) {
dbp->err(dbp, ret, "%s: DB->open", database);
goto err;
}
/* Acquire a cursor for the database. */
if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) {
dbp->err(dbp, ret, "DB->cursor");
goto err;
}
close_dbc = 1;
/* Initialize the key/data return pair. */
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
/* Walk through the database and print out the key/data pairs. */
while ((ret = dbcp->get(dbcp, &key, &data, DB_NEXT)) == 0)
printf("%.*s : %.*s\n",
(int)key.size, (char *)key.data,
(int)data.size, (char *)data.data);
if (ret != DB_NOTFOUND) {
dbp->err(dbp, ret, "DBcursor->get");
goto err;
}
err: if (close_dbc && (ret = dbcp->close(dbcp)) != 0)
dbp->err(dbp, ret, "DBcursor->close");
if (close_db && (ret = dbp->close(dbp, 0)) != 0)
fprintf(stderr,
"%s: DB->close: %s\n", progname, db_strerror(ret));
return (0);
}</pre>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_curput"></a>Storing records with a cursor</h3>
</div>
</div>
</div>
<p>
The <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> method stores records into the database using
a cursor. In general, <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> takes a key and inserts the
associated data into the database, at a location controlled by
a specified flag.
</p>
<p>
There are several flags that you can set to customize
storage:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<a href="../api_reference/C/dbcput.html#dbcput_DB_AFTER" class="olink">DB_AFTER</a>
</span>
</dt>
<dd>
Create a new record, immediately after the
record to which the cursor refers.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcput.html#dbcput_DB_BEFORE" class="olink">DB_BEFORE</a>
</span>
</dt>
<dd>
Create a new record, immediately before the
record to which the cursor refers.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class="olink">DB_CURRENT</a>
</span>
</dt>
<dd>
Replace the data part of the record to which
the cursor refers.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcput.html#dbcput_DB_KEYFIRST" class="olink">DB_KEYFIRST</a>
</span>
</dt>
<dd>
Create a new record as the first of the
duplicate records for the supplied key.
</dd>
<dt>
<span class="term">
<a href="../api_reference/C/dbcput.html#dbcput_DB_KEYLAST" class="olink">DB_KEYLAST</a>
</span>
</dt>
<dd>
Create a new record, as the last of the
duplicate records for the supplied key.
</dd>
</dl>
</div>
<p>
In all cases, the cursor is repositioned by a <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a>
operation to point to the newly inserted key/data pair in the
database.
</p>
<p>
The following is a code example showing a cursor storing
two data items in a database that supports duplicate data
items:
</p>
<a id="prog_am20"></a>
<pre class="programlisting">int
store(DB *dbp)
{
DBC *dbcp;
DBT key, data;
int ret;
/*
* The DB handle for a Btree database supporting duplicate data
* items is the argument; acquire a cursor for the database.
*/
if ((ret = dbp->cursor(dbp, NULL, &dbcp, 0)) != 0) {
dbp->err(dbp, ret, "DB->cursor");
goto err;
}
/* Initialize the key. */
memset(&key, 0, sizeof(key));
key.data = "new key";
key.size = strlen(key.data) + 1;
/* Initialize the data to be the first of two duplicate records. */
memset(&data, 0, sizeof(data));
data.data = "new key's data: entry #1";
data.size = strlen(data.data) + 1;
/* Store the first of the two duplicate records. */
if ((ret = dbcp->put(dbcp, &key, &data, DB_KEYFIRST)) != 0)
dbp->err(dbp, ret, "DB->cursor");
/* Initialize the data to be the second of two duplicate records. */
data.data = "new key's data: entry #2";
data.size = strlen(data.data) + 1;
/*
* Store the second of the two duplicate records. No duplicate
* record sort function has been specified, so we explicitly
* store the record as the last of the duplicate set.
*/
if ((ret = dbcp->put(dbcp, &key, &data, DB_KEYLAST)) != 0)
dbp->err(dbp, ret, "DB->cursor");
err: if ((ret = dbcp->close(dbcp)) != 0)
dbp->err(dbp, ret, "DBcursor->close");
return (0);
}</pre>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If you are using the Heap access method and you are
creating a new record in the database, then the key that
you provide to the <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> method should be empty. The
<a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> method will return the record's ID (RID) in the
key. The RID is automatically created for you when Heap
database records are created.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_curdel"></a>Deleting records with a cursor</h3>
</div>
</div>
</div>
<p>
The <a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a> method deletes records from the database using
a cursor. The <a href="../api_reference/C/dbcdel.html" class="olink">DBC->del()</a> method deletes the record to which the
cursor currently refers. In all cases, the cursor position is
unchanged after a delete.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_curdup"></a>Duplicating a cursor</h3>
</div>
</div>
</div>
<p>
Once a cursor has been initialized (for example, by a call
to <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>), it can be thought of as identifying a particular
location in a database. The <a href="../api_reference/C/dbcdup.html" class="olink">DBC->dup()</a> method permits an
application to create a new cursor that has the same locking
and transactional information as the cursor from which it is
copied, and which optionally refers to the same position in
the database.
</p>
<p>
In order to maintain a cursor position when an application
is using locking, locks are maintained on behalf of the cursor
until the cursor is closed. In cases when an application is
using locking without transactions, cursor duplication is
often required to avoid self-deadlocks. For further details,
refer to <a class="xref" href="lock_am_conv.html" title="Berkeley DB Transactional Data Store locking conventions">Berkeley DB Transactional Data
Store locking conventions</a>.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_join"></a>Equality Join</h3>
</div>
</div>
</div>
<p>
Berkeley DB supports "equality" (also known as "natural"),
joins on secondary indices. An equality join is a method of
retrieving data from a primary database using criteria stored
in a set of secondary indices. It requires the data be
organized as a primary database which contains the primary key
and primary data field, and a set of secondary indices. Each
of the secondary indices is indexed by a different secondary
key, and, for each key in a secondary index, there is a set of
duplicate data items that match the primary keys in the
primary database.
</p>
<p>
For example, let's assume the need for an application that
will return the names of stores in which one can buy fruit of
a given color. We would first construct a primary database
that lists types of fruit as the key item, and the store where
you can buy them as the data item:
</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Primary key:</th>
<th>Primary data:</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">apple</td>
<td align="left">Convenience Store</td>
</tr>
<tr>
<td align="left">blueberry</td>
<td align="left">Farmer's Market</td>
</tr>
<tr>
<td align="left">peach</td>
<td align="left">Shopway</td>
</tr>
<tr>
<td align="left">pear</td>
<td align="left">Farmer's Market</td>
</tr>
<tr>
<td align="left">raspberry</td>
<td align="left">Shopway</td>
</tr>
<tr>
<td align="left">strawberry</td>
<td align="left">Farmer's Market</td>
</tr>
</tbody>
</table>
</div>
<p>
We would then create a secondary index with the key
<span class="bold"><strong>color</strong></span>, and, as the data
items, the names of fruits of different colors.
</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Secondary key:</th>
<th>Secondary data:</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">blue</td>
<td align="left">blueberry</td>
</tr>
<tr>
<td align="left">red</td>
<td align="left">apple</td>
</tr>
<tr>
<td align="left">red</td>
<td align="left">raspberry</td>
</tr>
<tr>
<td align="left">red</td>
<td align="left">strawberry</td>
</tr>
<tr>
<td align="left">yellow</td>
<td align="left">peach</td>
</tr>
<tr>
<td align="left">yellow</td>
<td align="left">pear</td>
</tr>
</tbody>
</table>
</div>
<p>
This secondary index would allow an application to look up a
color, and then use the data items to look up the stores where
the colored fruit could be purchased. For example, by first
looking up <span class="bold"><strong>blue</strong></span>, the data
item <span class="bold"><strong>blueberry</strong></span> could be used
as the lookup key in the primary database, returning <span class="bold"><strong>Farmer's Market</strong></span>.
</p>
<p>
Your data must be organized in the following manner in order
to use the <a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method:
</p>
<div class="orderedlist">
<ol type="1">
<li>
The actual data should be stored in the database
represented by the <a href="../api_reference/C/db.html" class="olink">DB</a> object used to invoke this method.
Generally, this <a href="../api_reference/C/db.html" class="olink">DB</a> object is called the
<span class="emphasis"><em>primary</em></span>.
</li>
<li>
Secondary indices should be stored in separate
databases, whose keys are the values of the secondary
indices and whose data items are the primary keys
corresponding to the records having the designated
secondary key value. It is acceptable (and expected) that
there may be duplicate entries in the secondary indices.
<p>
These duplicate entries should be sorted for
performance reasons, although it is not required. For
more information see the <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_DUPSORT" class="olink">DB_DUPSORT</a> flag to the
<a href="../api_reference/C/dbset_flags.html" class="olink">DB->set_flags()</a> method.
</p></li>
</ol>
</div>
<p>
What the <a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method does is review a list of secondary
keys, and, when it finds a data item that appears as a data
item for all of the secondary keys, it uses that data item as
a lookup into the primary database, and returns the associated
data item.
</p>
<p>
If there were another secondary index that had as its key
the <span class="bold"><strong>cost</strong></span> of the fruit, a
similar lookup could be done on stores where inexpensive fruit
could be purchased:
</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Secondary key:</th>
<th>Secondary data:</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">expensive</td>
<td align="left">blueberry</td>
</tr>
<tr>
<td align="left">expensive</td>
<td align="left">peach</td>
</tr>
<tr>
<td align="left">expensive</td>
<td align="left">pear</td>
</tr>
<tr>
<td align="left">expensive</td>
<td align="left">strawberry</td>
</tr>
<tr>
<td align="left">inexpensive</td>
<td align="left">apple</td>
</tr>
<tr>
<td align="left">inexpensive</td>
<td align="left">pear</td>
</tr>
<tr>
<td align="left">inexpensive</td>
<td align="left">raspberry</td>
</tr>
</tbody>
</table>
</div>
<p>
The <a href="../api_reference/C/dbjoin.html" class="olink">DB->join()</a> method provides equality join functionality.
While not strictly cursor functionality, in that it is not a
method off a cursor handle, it is more closely related to the
cursor operations than to the standard <a href="../api_reference/C/db.html" class="olink">DB</a> operations.
</p>
<p>
It is also possible to do lookups based on multiple criteria
in a single operation. For example, it is possible to look up
fruits that are both red and expensive in a single operation.
If the same fruit appeared as a data item in both the color
and expense indices, then that fruit name would be used as the
key for retrieval from the primary index, and would then
return the store where expensive, red fruit could be
purchased.
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp472064"></a>Example</h4>
</div>
</div>
</div>
<p>
Consider the following three databases:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">personnel</span>
</dt>
<dd>
<div class="itemizedlist">
<ul type="disc">
<li>key = SSN</li>
<li>data = record containing name,
address, phone number, job
title</li>
</ul>
</div>
</dd>
<dt>
<span class="term">lastname</span>
</dt>
<dd>
<div class="itemizedlist">
<ul type="disc">
<li>key = lastname</li>
<li>data = SSN</li>
</ul>
</div>
</dd>
<dt>
<span class="term">jobs</span>
</dt>
<dd>
<div class="itemizedlist">
<ul type="disc">
<li>key = job title</li>
<li>data = SSN</li>
</ul>
</div>
</dd>
</dl>
</div>
<p>
Consider the following query:
</p>
<pre class="programlisting">Return the personnel records of all people named smith with the job
title manager.</pre>
<p>
This query finds are all the records in the primary
database (personnel) for whom the criteria <span class="bold"><strong>lastname=smith and job
title=manager</strong></span> is true.
</p>
<p>
Assume that all databases have been properly opened and
have the handles: pers_db, name_db, job_db. We also assume
that we have an active transaction to which the handle txn
refers.
</p>
<a id="prog_am21"></a>
<pre class="programlisting">DBC *name_curs, *job_curs, *join_curs;
DBC *carray[3];
DBT key, data;
int ret, tret;
name_curs = NULL;
job_curs = NULL;
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
if ((ret =
name_db->cursor(name_db, txn, &name_curs, 0)) != 0)
goto err;
key.data = "smith";
key.size = sizeof("smith");
if ((ret =
name_curs->get(name_curs, &key, &data, DB_SET)) != 0)
goto err;
if ((ret = job_db->cursor(job_db, txn, &job_curs, 0)) != 0)
goto err;
key.data = "manager";
key.size = sizeof("manager");
if ((ret =
job_curs->get(job_curs, &key, &data, DB_SET)) != 0)
goto err;
carray[0] = name_curs;
carray[1] = job_curs;
carray[2] = NULL;
if ((ret =
pers_db->join(pers_db, carray, &join_curs, 0)) != 0)
goto err;
while ((ret =
join_curs->get(join_curs, &key, &data, 0)) == 0) {
/* Process record returned in key/data. */
}
/*
* If we exited the loop because we ran out of records,
* then it has completed successfully.
*/
if (ret == DB_NOTFOUND)
ret = 0;
err:
if (join_curs != NULL &&
(tret = join_curs->close(join_curs)) != 0 && ret == 0)
ret = tret;
if (name_curs != NULL &&
(tret = name_curs->close(name_curs)) != 0 && ret == 0)
ret = tret;
if (job_curs != NULL &&
(tret = job_curs->close(job_curs)) != 0 && ret == 0)
ret = tret;
return (ret);</pre>
<p>
The name cursor is positioned at the beginning of the
duplicate list for <span class="bold"><strong>smith</strong></span>
and the job cursor is placed at the beginning of the
duplicate list for <span class="bold"><strong>manager</strong></span>.
The join cursor is returned from the join method. This code then
loops over the join cursor getting the personnel records of each one
until there are no more.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_count"></a>Data item count</h3>
</div>
</div>
</div>
<p>
Once a cursor has been initialized to refer to a particular
key in the database, it can be used to determine the number of
data items that are stored for any particular key. The
<a href="../api_reference/C/dbccount.html" class="olink">DBC->count()</a> method returns this number of data items. The
returned value is always one, unless the database supports
duplicate data items, in which case it may be any number of
items.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="am_curclose"></a>Cursor close</h3>
</div>
</div>
</div>
<p>
The <a href="../api_reference/C/dbcclose.html" class="olink">DBC->close()</a> method closes the <a href="../api_reference/C/dbc.html" class="olink">DBC</a> cursor, after which
the cursor may no longer be used. Although cursors are
implicitly closed when the database they point to are closed,
it is good programming practice to explicitly close cursors.
In addition, in transactional systems, cursors may not exist
outside of a transaction and so must be explicitly
closed.
</p>
</div>
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