100 lines
6.7 KiB
HTML
100 lines
6.7 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Use DataLinks" />
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<meta name="abstract" content="The DataLink data type is one of the basic building blocks for extending the types of data that can be stored in database files. The idea of a DataLink is that the actual data stored in the column is only a pointer to the object." />
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<meta name="DC.subject" content="data types, DataLinks" />
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<meta name="keywords" content="data types, DataLinks" />
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<meta name="DC.Relation" scheme="URI" content="rbafyusoocap.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Use DataLinks</title>
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</head>
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<body id="rbafydatalinks"><a name="rbafydatalinks"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Use DataLinks</h1>
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<div><p>The DataLink data type is one of the basic building blocks for
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extending the types of data that can be stored in database files. The idea
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of a DataLink is that the actual data stored in the column is only a pointer
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to the object.</p>
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<p>This object can be anything, an image file, a voice recording, a text file,
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and so on. The method used for resolving to the object is to store a Uniform
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Resource Locator (URL). This means that a row in a table can be used to contain
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information about the object in traditional data types, and the object itself
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can be referenced using the DataLink data type. The user can use SQL scalar
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functions to get back the path to the object and the server on which the object
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is stored (see Built-in functions in the SQL Reference). With the DataLink
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data type, there is a fairly loose relationship between the row and the object.
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For instance, deleting a row will sever the relationship to the object referenced
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by the DataLink, but the object itself might not be deleted.</p>
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<p>A table created with a DataLink column can be used to hold information
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about an object, without actually containing the object itself. This concept
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gives the user much more flexibility in the types of data that can be managed
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using a table. If, for instance, the user has thousands of video clips stored
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in the integrated file system of their server, they may want to use an SQL
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table to contain information about these video clips. But since the user
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already has the objects stored in a directory, they only want the SQL table
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to contain references to the objects, not the actual bytes of storage. A
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good solution is to use DataLinks. The SQL table uses traditional SQL data
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types to contain information about each clip, such as title, length, date,
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and so on. But the clip itself is referenced using a DataLink column. Each
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row in the table stores a URL for the object and an optional comment. Then
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an application that is working with the clips can retrieve the URL using SQL
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interfaces, and then use a browser or other playback software to work with
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the URL and display the video clip.</p>
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<p>There are several advantages of using this technique:</p>
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<ul><li>The integrated file system can store any type of stream file.</li>
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<li>The integrated file system can store extremely large objects, that does
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not fit into a character column, or perhaps even a LOB column.</li>
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<li>The hierarchical nature of the integrated file system is well-suited to
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organizing and working with the stream file objects.</li>
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<li>By leaving the bytes of the object outside the database and in the integrated
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file system, applications can achieve better performance by allowing the SQL
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runtime engine to handle queries and reports, and allowing the file system
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to handle streaming of video, displaying images, text, and so on.</li>
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</ul>
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<p>Using DataLinks also gives control over the objects while they are in "linked"
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status. A DataLink column can be created such that the referenced object
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cannot be deleted, moved, or renamed while there is a row in the SQL table
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that references that object. This object are considered linked. Once the
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row containing that reference is deleted, the object is unlinked. To understand
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this concept fully, one should know the levels of control that can be specified
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when creating a DataLink column.</p>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rbafydlnolink.htm">Link control levels in DataLinks</a></strong><br />
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You can create a DataLink column with different link controls.</li>
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<li class="ulchildlink"><strong><a href="rbafydlcommands.htm">Commands used for working with DataLinks</a></strong><br />
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Support for the DataLink data type can be broken down into three different components.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafyusoocap.htm" title="Most data types, such as INTEGER and CHARACTER, do not have any special processing characteristics. However, there are a few data types that require special functions or locators in order to use them.">Process special data types</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../db2/rbafzmstch2data.htm">Data types</a></div>
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</div>
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</div>
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</body>
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</html> |