1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Getting Records Using the Cursor</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="Cursors.html" title="Chapter��4.��Using Cursors" /> 11 <link rel="prev" href="Cursors.html" title="Chapter��4.��Using Cursors" /> 12 <link rel="next" href="PutEntryWCursor.html" title="Putting Records Using Cursors" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Getting Records Using the Cursor</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="Cursors.html">Prev</a>��</td> 22 <th width="60%" align="center">Chapter��4.��Using Cursors</th> 23 <td width="20%" align="right">��<a accesskey="n" href="PutEntryWCursor.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="Positioning"></a>Getting Records Using the Cursor</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <dl> 38 <dt> 39 <span class="sect2"> 40 <a href="Positioning.html#cursorsearch">Searching for Records</a> 41 </span> 42 </dt> 43 <dt> 44 <span class="sect2"> 45 <a href="Positioning.html#getdups">Working with Duplicate Records</a> 46 </span> 47 </dt> 48 </dl> 49 </div> 50 <p> 51 To iterate over database records, from the first record to 52 the last, simply open the cursor and then use the 53 54 55 <code class="methodname">Dbc::get()</code> 56 method. 57 <span>Note that you need to supply the 58 <code class="literal">DB_NEXT</code> flag to this method.</span> 59 For example: 60 </p> 61 <a id="cxx_cursor3"></a> 62 <pre class="programlisting">#include <db_cxx.h> 63 64... 65 66Db my_database(NULL, 0); 67Dbc *cursorp; 68 69try { 70 // Database open omitted for clarity 71 72 // Get a cursor 73 my_database.cursor(NULL, &cursorp, 0); 74 75 Dbt key, data; 76 int ret; 77 78 // Iterate over the database, retrieving each record in turn. 79 while ((ret = cursorp->get(&key, &data, DB_NEXT)) == 0) { 80 // Do interesting things with the Dbts here. 81 } 82 if (ret != DB_NOTFOUND) { 83 // ret should be DB_NOTFOUND upon exiting the loop. 84 // Dbc::get() will by default throw an exception if any 85 // significant errors occur, so by default this if block 86 // can never be reached. 87 } 88} catch(DbException &e) { 89 my_database.err(e.get_errno(), "Error!"); 90} catch(std::exception &e) { 91 my_database.errx("Error! %s", e.what()); 92} 93 94// Cursors must be closed 95if (cursorp != NULL) 96 cursorp->close(); 97 98my_database.close(0);</pre> 99 <p> 100 To iterate over the database from the last record to the first, use 101 <code class="literal">DB_PREV</code> instead of <code class="literal">DB_NEXT</code>: 102 </p> 103 <a id="cxx_cursor4"></a> 104 <pre class="programlisting">#include <db_cxx.h> 105 106... 107 108Db my_database(NULL, 0); 109Dbc *cursorp; 110 111try { 112 // Database open omitted for clarity 113 114 // Get a cursor 115 my_database.cursor(NULL, &cursorp, 0); 116 117 Dbt key, data; 118 int ret; 119 // Iterate over the database, retrieving each record in turn. 120 while ((ret = cursorp->get(&key, &data, DB_PREV)) == 0) { 121 // Do interesting things with the Dbts here. 122 } 123 if (ret != DB_NOTFOUND) { 124 // ret should be DB_NOTFOUND upon exiting the loop. 125 // Dbc::get() will by default throw an exception if any 126 // significant errors occur, so by default this if block 127 // can never be reached. 128 } 129} catch(DbException &e) { 130 my_database.err(e.get_errno(), "Error!"); 131} catch(std::exception &e) { 132 my_database.errx("Error! %s", e.what()); 133} 134 135// Cursors must be closed 136if (cursorp != NULL) 137 cursorp->close(); 138 139my_database.close(0);</pre> 140 <div class="sect2" lang="en" xml:lang="en"> 141 <div class="titlepage"> 142 <div> 143 <div> 144 <h3 class="title"><a id="cursorsearch"></a>Searching for Records</h3> 145 </div> 146 </div> 147 </div> 148 <p> 149 You can use cursors to search for database records. You can search based 150 on just a key, or you can search based on both the key and the data. 151 You can also perform partial matches if your database supports sorted 152 duplicate sets. In all cases, the key and data parameters of these 153 methods are filled with the key and data values of the database record 154 to which the cursor is positioned as a result of the search. 155 </p> 156 <p> 157 Also, if the search fails, then cursor's state is left unchanged 158 and 159 160 <code class="literal">DB_NOTFOUND</code> 161 is returned. 162 163 164 </p> 165 <p> 166 To use a cursor to search for a record, use 167 168 <span>Dbt::get()<code class="methodname"></code>.</span> 169 When you use this method, you can provide the following flags: 170 </p> 171 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 172 <h3 class="title">Note</h3> 173 <p> 174 Notice in the following list that the cursor flags use the 175 keyword <code class="literal">SET</code> when the cursor examines just the key 176 portion of the records (in this case, the cursor is set to the 177 record whose key matches the value provided to the cursor). 178 Moreover, when the cursor uses the keyword <code class="literal">GET</code>, 179 then the cursor is positioned to both the key 180 <span class="emphasis"><em>and</em></span> the data values provided to the cursor. 181 </p> 182 <p> 183 Regardless of the keyword you use to get a record with a cursor, the 184 cursor's key and data 185 186 <span><code class="classname">Dbt</code>s</span> 187 are filled with the data retrieved from the record to which the 188 cursor is positioned. 189 </p> 190 </div> 191 <div class="itemizedlist"> 192 <ul type="disc"> 193 <li> 194 <p> 195 196 <code class="literal">DB_SET</code> 197 </p> 198 <p> 199 Moves the cursor to the first record in the database with 200 the specified key. 201 </p> 202 </li> 203 <li> 204 <p> 205 206 <code class="literal">DB_SET_RANGE</code> 207 </p> 208 <p> 209 <span>Identical to 210 <code class="literal">DB_SET</code> 211 212 unless you are using the BTree access. In this case, the cursor 213 moves</span> 214 215 216 to the first record in the database whose 217 key is greater than or equal to the specified key. This comparison 218 is determined by the 219 220 <span>comparison function</span> 221 that you provide for the database. If no 222 223 <span>comparison function</span> 224 is provided, then the default 225 226 lexicographical sorting is used. 227 </p> 228 <p> 229 For example, suppose you have database records that use the 230 following 231 <span>Strings</span> 232 233 as keys: 234 </p> 235 <pre class="programlisting">Alabama 236Alaska 237Arizona</pre> 238 <p> 239 Then providing a search key of <code class="literal">Alaska</code> moves the 240 cursor to the second key noted above. Providing a key of 241 <code class="literal">Al</code> moves the cursor to the first key (<code class="literal">Alabama</code>), providing 242 a search key of <code class="literal">Alas</code> moves the cursor to the second key 243 (<code class="literal">Alaska</code>), and providing a key of <code class="literal">Ar</code> moves the 244 cursor to the last key (<code class="literal">Arizona</code>). 245 </p> 246 </li> 247 <li> 248 <p> 249 250 <code class="literal">DB_GET_BOTH</code> 251 </p> 252 <p> 253 Moves the cursor to the first record in the database that uses 254 the specified key and data. 255 </p> 256 </li> 257 <li> 258 <p> 259 260 <code class="literal">DB_GET_BOTH_RANGE</code> 261 </p> 262 <p> 263 Moves the cursor to the first record in the database whose key matches the specified 264 key and whose data is 265 greater than or equal to the specified data. If the database supports 266 duplicate records, then on matching the key, the cursor is moved to 267 the duplicate record with the smallest data that is greater than or 268 equal to the specified data. 269 </p> 270 <p> 271 For example, 272 273 <span>suppose your database uses BTree 274 and it has </span> 275 database records that use the following key/data pairs: 276 </p> 277 <pre class="programlisting">Alabama/Athens 278Alabama/Florence 279Alaska/Anchorage 280Alaska/Fairbanks 281Arizona/Avondale 282Arizona/Florence </pre> 283 <p>then providing:</p> 284 <div class="informaltable"> 285 <table border="1" width="80%"> 286 <colgroup> 287 <col /> 288 <col /> 289 <col /> 290 </colgroup> 291 <thead> 292 <tr> 293 <th>a search key of ...</th> 294 <th>and a search data of ...</th> 295 <th>moves the cursor to ...</th> 296 </tr> 297 </thead> 298 <tbody> 299 <tr> 300 <td>Alaska</td> 301 <td>Fa</td> 302 <td>Alaska/Fairbanks</td> 303 </tr> 304 <tr> 305 <td>Arizona</td> 306 <td>Fl</td> 307 <td>Arizona/Florence</td> 308 </tr> 309 <tr> 310 <td>Alaska</td> 311 <td>An</td> 312 <td>Alaska/Anchorage</td> 313 </tr> 314 </tbody> 315 </table> 316 </div> 317 </li> 318 </ul> 319 </div> 320 <p> 321 For example, assuming a database containing sorted duplicate records of 322 U.S. States/U.S Cities key/data pairs (both as 323 <span>Strings),</span> 324 325 then the following code fragment can be used to position the cursor 326 to any record in the database and print its key/data values: 327 328 </p> 329 <a id="cxx_cursor5"></a> 330 <pre class="programlisting">#include <db_cxx.h> 331#include <string.h> 332 333... 334 335Db my_database(NULL, 0); 336Dbc *cursorp; 337 338try { 339 // database open omitted for clarity 340 341 // Get a cursor 342 my_database.cursor(NULL, &cursorp, 0); 343 344 // Search criteria 345 char *search_key = "Alaska"; 346 char *search_data = "Fa"; 347 348 // Set up our DBTs 349 Dbt key(search_key, strlen(search_key) + 1); 350 Dbt data(search_data, strlen(search_data) + 1); 351 352 // Position the cursor to the first record in the database whose 353 // key matches the search key and whose data begins with the search 354 // data. 355 int ret = cursorp->get(&key, &data, DB_GET_BOTH_RANGE); 356 if (!ret) { 357 // Do something with the data 358 } 359} catch(DbException &e) { 360 my_database.err(e.get_errno(), "Error!"); 361} catch(std::exception &e) { 362 my_database.errx("Error! %s", e.what()); 363} 364 365// Close the cursor 366if (cursorp != NULL) 367 cursorp->close(); 368 369// Close the database 370my_database.close(0); </pre> 371 </div> 372 <div class="sect2" lang="en" xml:lang="en"> 373 <div class="titlepage"> 374 <div> 375 <div> 376 <h3 class="title"><a id="getdups"></a>Working with Duplicate Records</h3> 377 </div> 378 </div> 379 </div> 380 <p> 381 A record is a duplicate of another record if the two records share the 382 same key. For duplicate records, only the data portion of the record is unique. 383 </p> 384 <p> 385 Duplicate records are supported only for the BTree or Hash access methods. 386 For information on configuring your database to use duplicate records, 387 see <a class="xref" href="btree.html#duplicateRecords" title="Allowing Duplicate Records">Allowing Duplicate Records</a>. 388 </p> 389 <p> 390 If your database supports duplicate records, then it can potentially 391 contain multiple records that share the same key. 392 393 394 395 <span>By default, normal database 396 get operations will only return the first such record in a set 397 of duplicate records. Typically, subsequent duplicate records are 398 accessed using a cursor. 399 </span> 400 401 The following 402 403 404 <span><code class="methodname">Dbc::get()</code> flags</span> 405 are interesting when working with databases that support duplicate records: 406 </p> 407 <div class="itemizedlist"> 408 <ul type="disc"> 409 <li> 410 <p> 411 412 <span> 413 <code class="literal">DB_NEXT</code>, 414 <code class="literal">DB_PREV</code> 415 </span> 416 </p> 417 <p> 418 Shows the next/previous record in the database, regardless of 419 whether it is a duplicate of the current record. For an example of 420 using these methods, see <a class="xref" href="Positioning.html" title="Getting Records Using the Cursor">Getting Records Using the Cursor</a>. 421 </p> 422 </li> 423 <li> 424 <p> 425 426 <code class="literal">DB_GET_BOTH_RANGE</code> 427 </p> 428 <p> 429 Useful for seeking the cursor to a specific record, regardless of 430 whether it is a duplicate record. See <a class="xref" href="Positioning.html#cursorsearch" title="Searching for Records">Searching for Records</a> for more 431 information. 432 </p> 433 </li> 434 <li> 435 <p> 436 437 <span> 438 <code class="literal">DB_NEXT_NODUP</code>, 439 <code class="literal">DB_PREV_NODUP</code> 440 </span> 441 </p> 442 <p> 443 Gets the next/previous non-duplicate record in the database. This 444 allows you to skip over all the duplicates in a set of duplicate 445 records. If you call 446 447 <span> 448 449 <code class="methodname">Dbc::get()</code> 450 with <code class="literal">DB_PREV_NODUP</code>, 451 </span> 452 then the cursor is positioned to the last record for the previous 453 key in the database. For example, if you have the following records 454 in your database: 455 </p> 456 <pre class="programlisting">Alabama/Athens 457Alabama/Florence 458Alaska/Anchorage 459Alaska/Fairbanks 460Arizona/Avondale 461Arizona/Florence</pre> 462 <p> 463 and your cursor is positioned to <code class="literal">Alaska/Fairbanks</code>, 464 and you then call 465 466 <span> 467 468 <code class="methodname">Dbc::get()</code> 469 with <code class="literal">DB_PREV_NODUP</code>, 470 </span> 471 then the cursor is positioned to Alabama/Florence. Similarly, if 472 you call 473 474 <span> 475 476 <code class="methodname">Dbc::get()</code> 477 with <code class="literal">DB_NEXT_NODUP</code>, 478 </span> 479 480 then the cursor is positioned to the first record corresponding to 481 the next key in the database. 482 </p> 483 <p> 484 If there is no next/previous key in the database, then 485 486 <code class="literal">DB_NOTFOUND</code> 487 is returned, and the cursor is left unchanged. 488 </p> 489 </li> 490 <li> 491 <p> 492 493 <code class="literal">DB_NEXT_DUP</code> 494 </p> 495 <p> 496 497 Gets the 498 499 <span>next</span> 500 record that shares the current key. If the 501 cursor is positioned at the last record in the duplicate set and 502 you call 503 504 <span> 505 506 <code class="methodname">Dbc::get()</code> 507 with <code class="literal">DB_NEXT_DUP</code>, 508 </span> 509 510 then 511 512 <code class="literal">DB_NOTFOUND</code> 513 is returned and the cursor is left unchanged. 514 515 </p> 516 </li> 517 </ul> 518 </div> 519 <p> 520 For example, the following code fragment positions a cursor to a key 521 522 523 524 <span>and displays it and all its 525 duplicates.</span> 526 527 528 </p> 529 <a id="cxx_cursor6"></a> 530 <pre class="programlisting">#include <db_cxx.h> 531#include <string.h> 532 533... 534 535char *search_key = "Al"; 536 537Db my_database(NULL, 0); 538Dbc *cursorp; 539 540try { 541 // database open omitted for clarity 542 543 // Get a cursor 544 my_database.cursor(NULL, &cursorp, 0); 545 546 // Set up our DBTs 547 Dbt key(search_key, strlen(search_key) + 1); 548 Dbt data; 549 550 // Position the cursor to the first record in the database whose 551 // key and data begin with the correct strings. 552 int ret = cursorp->get(&key, &data, DB_SET); 553 while (ret != DB_NOTFOUND) { 554 std::cout << "key: " << (char *)key.get_data() 555 << "data: " << (char *)data.get_data()<< std::endl; 556 ret = cursorp->get(&key, &data, DB_NEXT_DUP); 557 } 558} catch(DbException &e) { 559 my_database.err(e.get_errno(), "Error!"); 560} catch(std::exception &e) { 561 my_database.errx("Error! %s", e.what()); 562} 563 564// Close the cursor 565if (cursorp != NULL) 566 cursorp->close(); 567 568// Close the database 569my_database.close(0); </pre> 570 </div> 571 </div> 572 <div class="navfooter"> 573 <hr /> 574 <table width="100%" summary="Navigation footer"> 575 <tr> 576 <td width="40%" align="left"><a accesskey="p" href="Cursors.html">Prev</a>��</td> 577 <td width="20%" align="center"> 578 <a accesskey="u" href="Cursors.html">Up</a> 579 </td> 580 <td width="40%" align="right">��<a accesskey="n" href="PutEntryWCursor.html">Next</a></td> 581 </tr> 582 <tr> 583 <td width="40%" align="left" valign="top">Chapter��4.��Using Cursors��</td> 584 <td width="20%" align="center"> 585 <a accesskey="h" href="index.html">Home</a> 586 </td> 587 <td width="40%" align="right" valign="top">��Putting Records Using Cursors</td> 588 </tr> 589 </table> 590 </div> 591 </body> 592</html> 593