129 lines
7.9 KiB
HTML
129 lines
7.9 KiB
HTML
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<?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="reference" />
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<meta name="DC.Title" content="Delete from tables with referential constraints" />
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<meta name="abstract" content="If a table has a primary key but no dependents, the DELETE statement operates as it does without referential constraints. The same is true if a table has only foreign keys, but no primary key. If a table has a primary key and dependent tables, DELETE deletes or updates rows according to the delete rules specified." />
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<meta name="description" content="If a table has a primary key but no dependents, the DELETE statement operates as it does without referential constraints. The same is true if a table has only foreign keys, but no primary key. If a table has a primary key and dependent tables, DELETE deletes or updates rows according to the delete rules specified." />
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<meta name="DC.subject" content="DELETE statement, deleting from tables, constraint, referential, delete rules" />
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<meta name="keywords" content="DELETE statement, deleting from tables, constraint, referential, delete rules" />
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<meta name="DC.Relation" scheme="URI" content="rbafydelete.htm" />
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<meta name="DC.Relation" scheme="URI" content="rbafydeleterules.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>Delete from tables with referential constraints</title>
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</head>
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<body id="rbafyrfdeleting"><a name="rbafyrfdeleting"><!-- --></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">Delete from tables with referential constraints</h1>
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<div><p>If a table has a primary key but no dependents, the DELETE statement
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operates as it does without referential constraints. The same is true if a
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table has only foreign keys, but no primary key. If a table has a primary
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key and dependent tables, DELETE deletes or updates rows according
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to the delete rules specified.</p>
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<div class="section"><p>All delete rules of all affected relationships must be satisfied
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in order for the delete operation to succeed. If a referential constraint
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is violated, the DELETE fails. </p>
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</div>
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<div class="section"><p>The action to be taken on dependent tables when a DELETE is performed
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on a parent table depends on the delete rule specified for the referential
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constraint. If no delete rule was defined, the DELETE NO ACTION rule is used.
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</p>
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<dl><dt class="dlterm">DELETE NO ACTION </dt>
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<dd>Specifies that the row in the parent table can be deleted if no other
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row depends on it. If a dependent row exists in the relationship, the DELETE
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fails. The check for dependent rows is performed at the end of the statement.</dd>
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<dt class="dlterm">DELETE RESTRICT </dt>
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<dd>Specifies that the row in the parent table can be deleted if no other
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row depends on it. If a dependent row exists in the relationship, the DELETE
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fails. The check for dependent rows is performed immediately. <p>For example,
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you cannot delete a department from the department table if it is still responsible
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for some project that is described by a dependent row in the project table.</p>
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</dd>
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<dt class="dlterm">DELETE CASCADE </dt>
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<dd>Specifies that first the designated rows in the parent table are deleted.
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Then, the dependent rows are deleted. <p>For example, you can delete a department
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by deleting its row in the department table. Deleting the row from the department
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table also deletes: </p>
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<ul><li>The rows for all departments that report to it</li>
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<li>All departments that report to those departments and so forth.</li>
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</ul>
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</dd>
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<dt class="dlterm">DELETE SET NULL</dt>
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<dd>Specifies that each nullable column of the foreign key in each dependent
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row is set to its default value. This means that the column is only set to
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its default value if it is a member of a foreign key that references the row
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being deleted. Only the dependent rows that are immediate descendents are
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affected.</dd>
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<dt class="dlterm">DELETE SET DEFAULT </dt>
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<dd>Specifies that each column of the foreign key in each dependent row is
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set to its default value. This means that the column is only set to its default
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value if it is a member of a foreign key that references the row being deleted.
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Only the dependent rows that are immediate descendants are affected. <p>For
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example, you can delete an employee from the employee table (EMPLOYEE) even
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if the employee manages some department. In that case, the value of MGRNO
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for each employee who reported to the manager is set to blanks in the department
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table (DEPARTMENT). If some other default value was specified on the create
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of the table, that value is used.</p>
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<p>This is due to the REPORTS_TO_EXISTS
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constraint defined for the department table.</p>
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</dd>
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</dl>
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</div>
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<div class="section"><p>If a descendent table has a delete rule of RESTRICT or NO ACTION
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and a row is found such that a descendant row cannot be deleted, the entire
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DELETE fails.</p>
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</div>
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<div class="section"><p>When running this statement with a program, the number of rows
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deleted is returned in SQLERRD(3) in the SQLCA. This number includes only
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the number of rows deleted in the table specified in the DELETE statement.
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It does not include those rows deleted according to the CASCADE rule. SQLERRD(5)
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in the SQLCA contains the number of rows that were affected by referential
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constraints in all tables. The SQLERRD(3) value is also available from the
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ROW_COUNT item in the GET DIAGNOSTICS statement. The SQLERRD(5) value is
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available from the DB2_ROW_COUNT_SECONDARY item.</p>
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</div>
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<div class="section"><p>The subtle difference between RESTRICT and NO ACTION rules is
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easiest seen when looking at the interaction of triggers and referential constraints.
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Triggers can be defined to fire either before or after an operation (a DELETE
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statement, in this case). A <em>before trigger</em> fires before the DELETE
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is performed and therefore before any checking of constraints. An <em>after
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trigger</em> is fired after the DELETE is performed, and after a constraint
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rule of RESTRICT (where checking is performed immediately), but before a constraint
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rule of NO ACTION (where checking is performed at the end of the statement).
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The triggers and rules occur in the following order: </p>
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</div>
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<div class="section"> <ol><li>A <em>before trigger</em> is fired before the DELETE and before a constraint
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rule of RESTRICT or NO ACTION.</li>
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<li>An <em>after trigger</em> is fired after a constraint rule of RESTRICT,
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but before a NO ACTION rule.</li>
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</ol>
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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="rbafydeleterules.htm">Example: DELETE cascade rule</a></strong><br />
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</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="rbafydelete.htm" title="To remove rows from a table, use the DELETE statement.">Remove rows from a table using the DELETE statement</a></div>
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</div>
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</div>
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</body>
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</html>
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