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<h1>Query By Example</h1><script>
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<p>When using <i>Query By Example</i>(QBE) you provide db4o with a template
object. db4o will return all of the objects which match all non-default field
values. This is done via reflecting all of the fields and building a query
expression where all non-default-value fields are combined with AND expressions.
Here's a simple example:</p>
<span name="cs_wiki_filter" csw_filters="cs">
<div class="FormattedSourceCode"><div class="fscHeader"><span class="fscFileName">PersistentExample.cs: <span class="fscMemberName">RetrievePilotByName</span></span></div><div class="fscCode"><pre ID="ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68"><div><!--
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--><span style="color: #008080;">1</span><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/None.gif" align="top"/><span style="color: #0000FF;">public</span><span style="color: #000000;"> </span><span style="color: #0000FF;">static</span><span style="color: #000000;"> </span><span style="color: #0000FF;">void</span><span style="color: #000000;"> RetrievePilotByName(IObjectContainer db)
</span><span style="color: #008080;">2</span><span style="color: #000000;"><img id="ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Open_Image" src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ExpandedBlockStart.gif" align="top" onClick="this.style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Open_Text').style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Closed_Image').style.display='inline'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Closed_Text').style.display='inline';"/><img id="ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Closed_Image" src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ContractedBlock.gif" align="top" style="display: none;" onClick="this.style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Closed_Text').style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Open_Image').style.display='inline'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Open_Text').style.display='inline';"/> </span><span id="ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Closed_Text" style="border: solid 1px #808080; background-color: #FFFFFF; display: none;">...</span><span id="ResourcesReferenceObject_LifecycleQueryingqueriesCS.ZipsPersistentExample.Cs_PersistentExample.Cs_RetrievePilotByName_399_68_62_181_Open_Text"><span style="color: #000000;">{
</span><span style="color: #008080;">3</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> Pilot proto </span><span style="color: #000000;">=</span><span style="color: #000000;"> </span><span style="color: #0000FF;">new</span><span style="color: #000000;"> Pilot(</span><span style="color: #800000;">"</span><span style="color: #800000;">Michael Schumacher</span><span style="color: #800000;">"</span><span style="color: #000000;">, </span><span style="color: #800080;">0</span><span style="color: #000000;">);
</span><span style="color: #008080;">4</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> IObjectSet result </span><span style="color: #000000;">=</span><span style="color: #000000;"> db.Get(proto);
</span><span style="color: #008080;">5</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> ListResult(result);
</span><span style="color: #008080;">6</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ExpandedBlockEnd.gif" align="top"/> }</span></span></div></pre></div></div>
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<span name="cs_wiki_filter" csw_filters="vb">
<div class="FormattedSourceCode"><div class="fscHeader"><span class="fscFileName">PersistentExample.vb: <span class="fscMemberName">RetrievePilotByName</span></span></div><div class="fscCode"><pre ID="ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68"><div><!--
Code highlighting produced by Actipro CodeHighlighter (freeware)
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--><span style="color: #008080;">1</span><img id="ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Open_Image" src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ExpandedBlockStart.gif" align="top" onClick="this.style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Open_Text').style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Closed_Image').style.display='inline'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Closed_Text').style.display='inline';"/><img id="ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Closed_Image" src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ContractedBlock.gif" align="top" style="display: none;" onClick="this.style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Closed_Text').style.display='none'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Open_Image').style.display='inline'; document.getElementById('ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Open_Text').style.display='inline';"/><span style="color: #0000FF;">Private</span><span style="color: #000000;"> </span><span style="color: #0000FF;">Shared</span><span style="color: #000000;"> </span><span id="ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Closed_Text" style="border: solid 1px #808080; background-color: #FFFFFF; display: none;">Sub RetrievePilotByName()</span><span id="ResourcesReferenceObject_LifecycleQueryingqueriesVB.ZipsPersistentExample.Vb_PersistentExample.Vb_RetrievePilotByName_525_68_15_236_Open_Text"><span style="color: #0000FF;">Sub</span><span style="color: #000000;"> RetrievePilotByName(</span><span style="color: #0000FF;">ByVal</span><span style="color: #000000;"> db </span><span style="color: #0000FF;">As</span><span style="color: #000000;"> IObjectContainer)
</span><span style="color: #008080;">2</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> </span><span style="color: #0000FF;">Dim</span><span style="color: #000000;"> proto </span><span style="color: #0000FF;">As</span><span style="color: #000000;"> Pilot </span><span style="color: #000000;">=</span><span style="color: #000000;"> </span><span style="color: #0000FF;">New</span><span style="color: #000000;"> Pilot(</span><span style="color: #800000;">"</span><span style="color: #800000;">Michael Schumacher</span><span style="color: #800000;">"</span><span style="color: #000000;">, </span><span style="color: #800080;">0</span><span style="color: #000000;">)
</span><span style="color: #008080;">3</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> </span><span style="color: #0000FF;">Dim</span><span style="color: #000000;"> result </span><span style="color: #0000FF;">As</span><span style="color: #000000;"> IObjectSet </span><span style="color: #000000;">=</span><span style="color: #000000;"> db.Get(proto)
</span><span style="color: #008080;">4</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/InBlock.gif" align="top"/> ListResult(result)
</span><span style="color: #008080;">5</span><span style="color: #000000;"><img src="../../../Utility/ActiPro.CodeHighligher/OutliningIndicators/ExpandedBlockEnd.gif" align="top"/> </span><span style="color: #0000FF;">End Sub</span></span></div></pre></div></div>
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<p>Querying
this way has some obvious limitations:</p>
<ul>
<li>db4o must reflect all members of
your example object.</li>
<li>You cannot perform advanced query expressions. (AND,
OR, NOT, etc.)</li>
<li>You cannot constrain on values like 0 (integers), "" (empty
strings), or nulls (reference types) because they would be interpreted as
unconstrained.</li>
<li>You need to be able to create <a href="query_by_example/qbe_limitations.html" class="wikiLink">objects without initialized
fields</a>. That means you can not initialize fields where they are declared. <br></li>
<li>You need a <a href="query_by_example/qbe_limitations.html" class="wikiLink">constructor</a> to create objects
without initialized fields.</li>
</ul>
<p>For more information see <a href="query_by_example/qbe_limitations.html" class="wikiLink">QBE Limitations</a>. To get around all of these constraints, db4o
provides the <a href="native_queries.html" class="wikiLink">Native Query</a> (NQ) system.</p></div>
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This revision (7) was last Modified 2007-10-18T06:39:16 by Tetyana.
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