103 lines
6.4 KiB
HTML
103 lines
6.4 KiB
HTML
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<meta name="DC.Title" content="Referential integrity and tables" />
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<meta name="abstract" content="Referential integrity is the condition of a set of tables in a database in which all references from one table to another are valid." />
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<title>Referential integrity and tables</title>
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<body id="rbafyrefinteg"><a name="rbafyrefinteg"><!-- --></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">Referential integrity and tables</h1>
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<div><p>Referential integrity is the condition of a set of tables in a
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database in which all references from one table to another are valid. </p>
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<div class="section"><p>Consider
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the following example: </p>
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<ul><li>CORPDATA.EMPLOYEE serves as a master list of employees.</li>
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<li>CORPDATA.DEPARTMENT acts as a master list of all valid department numbers.</li>
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<li>CORPDATA.EMP_ACT provides a master list of activities performed for projects.</li>
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</ul>
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<p>Other tables refer to the same entities described in these tables.
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When a table contains data for which there is a master list, that data should
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actually appear in the master list, or the reference is not valid. The table
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that contains the master list is the <dfn class="term">parent table</dfn>, and the table
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that refers to it is a <dfn class="term">dependent table</dfn>. When the references from
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the dependent table to the parent table are valid, the condition of the set
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of tables is called <dfn class="term">referential integrity</dfn>.</p>
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</div>
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<div class="section"><p>Stated another way, referential integrity is the state of a database
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in which all values of all foreign keys are valid. Each value of the foreign
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key must also exist in the parent key or be null. This definition of referential
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integrity requires an understanding of the following terms: </p>
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<ul><li>A <dfn class="term">unique key</dfn> is a column or set of columns in a table which
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uniquely identify a row. Although a table can have several unique keys, no
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two rows in a table can have the same unique key value.</li>
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<li>A <dfn class="term">primary key</dfn> is a unique key that does not allow nulls.
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A table cannot have more than one primary key.</li>
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<li>A <dfn class="term">parent key</dfn> is either a unique key or a primary key which
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is referenced in a referential constraint.</li>
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<li>A <dfn class="term">foreign key</dfn> is a column or set of columns whose values
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must match those of a parent key. If any column value used to build the foreign
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key is null, then the rule does not apply.</li>
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<li>A <dfn class="term">parent table</dfn> is a table that contains the parent key.</li>
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<li>A <dfn class="term">dependent table</dfn> is the table that contains the foreign
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key.</li>
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<li>A <dfn class="term">descendent table</dfn> is a table that is a dependent table or
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a descendent of a dependent table.</li>
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</ul>
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</div>
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<div class="section"><p>Enforcement of referential integrity prevents the violation of
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the rule that states that every non-null foreign key must have
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a matching parent key.</p>
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</div>
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<div class="section"><p>SQL supports the referential integrity concept with the CREATE
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TABLE and ALTER TABLE statements. </p>
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</div>
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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="rbafyaddor.htm">Add or drop referential constraints</a></strong><br />
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Constraints are rules that ensure that references from one table, a dependent table, to data in another table, the parent table, are valid. You use referential constraints to ensure referential integrity.</li>
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<li class="ulchildlink"><strong><a href="rbafyaddrcon.htm">Add referential constraints</a></strong><br />
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The rule that every department number shown in the sample employee table must appear in the department table is a referential constraint.</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="rbafysqlpctablecreate.htm" title="A table can be visualized as a two-dimensional arrangement of data consisting of rows and columns.">Create a table</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rbafysamptblx.htm" title="This topic contains the sample tables referred to and used in this topic and the SQL Reference topic collection.">DB2 UDB for iSeries sample tables</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/rbafzmsthctabl.htm">CREATE TABLE</a></div>
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<div><a href="../db2/rbafzmstatabl.htm">ALTER TABLE</a></div>
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</div>
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</div>
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</body>
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