docs/gsg/C/secondaryDelete.html

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  21           <th colspan="3" align="center">Deleting Secondary Database Records</th>
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  24           <td width="20%" align="left"><a accesskey="p" href="readSecondary.html">Prev</a> </td>
  25           <th width="60%" align="center">Chapter 5. Secondary Databases</th>
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  35             <h2 class="title" style="clear: both"><a id="secondaryDelete"></a>Deleting Secondary Database Records</h2>
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  39       <p>
  40         In general, you
  41
  42             <span>will</span>
  43         not modify a secondary database directly. In
  44         order to modify a secondary database, you should modify the primary
  45         database and simply allow DB to manage the secondary modifications for you.
  46     </p>
  47       <p>
  48         However, as a convenience, you can delete
  49
  50             <span>secondary database</span>
  51         records directly. Doing so causes the associated primary key/data pair to be deleted.
  52         This in turn causes DB to delete all
  53
  54             <span>secondary database</span>
  55         records that reference the primary record.
  56     </p>
  57       <p>
  58         You can use the
  59
  60             <code class="methodname">DB-&gt;del()</code>
  61
  62         method to delete a secondary database record.
  63
  64
  65
  66         <span>Note that if your
  67             <span>secondary database</span>
  68
  69         contains duplicate records, then deleting a record from the set of
  70         duplicates causes all of the duplicates to be deleted as well.
  71         </span>
  72
  73     </p>
  74       <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
  75         <h3 class="title">Note</h3>
  76         <p>
  77
  78       <span>
  79         You can delete a secondary database record using the previously
  80         described mechanism
  81       </span>
  82       only if the primary database is opened for write access.
  83       </p>
  84       </div>
  85       <p>For example:</p>
  86       <a id="c_index7"></a>
  87       <pre class="programlisting">#include &lt;db.h&gt;
  88 #include &lt;string.h&gt;
  89
  90 ...
  91
  92 DB *dbp, *sdbp;    /* Primary and secondary DB handles */
  93 DBT key;           /* DBTs used for the delete */
  94 int ret;           /* Function return value */
  95 char *search_name = "John Doe"; /* Name to delete */
  96
  97 /* Primary */
  98 ret = db_create(&amp;dbp, NULL, 0);
  99 if (ret != 0) {
 100   /* Error handling goes here */
 101 }
 102
 103 /* Secondary */
 104 ret = db_create(&amp;sdbp, NULL, 0);
 105 if (ret != 0) {
 106   /* Error handling goes here */
 107 }
 108
 109 /* Usually we want to support duplicates for secondary databases */
 110 ret = sdbp-&gt;set_flags(sdbp, DB_DUPSORT);
 111 if (ret != 0) {
 112   /* Error handling goes here */
 113 }
 114
 115 /* open the primary database */
 116 ret = dbp-&gt;open(dbp,        /* DB structure pointer */
 117                 NULL,       /* Transaction pointer */
 118                 "my_db.db", /* On-disk file that holds the database.
 119                              * Required. */
 120                 NULL,       /* Optional logical database name */
 121                 DB_BTREE,   /* Database access method */
 122                 0,          /* Open flags */
 123                 0);         /* File mode (using defaults) */
 124 if (ret != 0) {
 125   /* Error handling goes here */
 126 }
 127
 128 /* open the secondary database */
 129 ret = sdbp-&gt;open(sdbp,          /* DB structure pointer */
 130                  NULL,          /* Transaction pointer */
 131                  "my_secdb.db", /* On-disk file that holds the database.
 132                                  * Required. */
 133                  NULL,          /* Optional logical database name */
 134                  DB_BTREE,      /* Database access method */
 135                  0,             /* Open flags */
 136                  0);            /* File mode (using defaults) */
 137 if (ret != 0) {
 138   /* Error handling goes here */
 139 }
 140
 141 /* Now associate the secondary to the primary */
 142 dbp-&gt;associate(dbp,            /* Primary database */
 143                NULL,           /* TXN id */
 144                sdbp,           /* Secondary database */
 145                get_sales_rep,  /* Callback used for key creation. */
 146                0);             /* Flags */
 147
 148 /*
 149  * Zero out the DBT before using it.
 150  */
 151 memset(&amp;key, 0, sizeof(DBT));
 152
 153 key.data = search_name;
 154 key.size = strlen(search_name) + 1;
 155
 156 /* Now delete the secondary record. This causes the associated primary
 157  * record to be deleted. If any other secondary databases have secondary
 158  * records referring to the deleted primary record, then those secondary
 159  * records are also deleted.
 160  */
 161  sdbp-&gt;del(sdbp, NULL, &amp;key, 0); </pre>
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