98 lines
6.3 KiB
HTML
98 lines
6.3 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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<meta name="DC.Type" content="reference" />
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<meta name="DC.Title" content="Add referential constraints" />
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<meta name="abstract" content="The rule that every department number shown in the sample employee table must appear in the department table is a referential constraint." />
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<meta name="description" content="The rule that every department number shown in the sample employee table must appear in the department table is a referential constraint." />
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<meta name="DC.subject" content="examples, adding constraints, CREATE TABLE statement, referential constraints, ALTER TABLE statement, constraint, referential, creating tables" />
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<meta name="keywords" content="examples, adding constraints, CREATE TABLE statement, referential constraints, ALTER TABLE statement, constraint, referential, creating tables" />
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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>Add referential constraints</title>
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</head>
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<body id="rbafyaddrcon"><a name="rbafyaddrcon"><!-- --></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">Add referential constraints</h1>
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<div><p>The rule that every department number shown in the sample employee
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table must appear in the department table is a referential constraint.</p>
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<div class="section"><p>This constraint ensures that every employee belongs to an existing
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department. The following SQL statements create the CORPDATA.DEPARTMENT and
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CORPDATA.EMPLOYEE tables with those constraint relationships defined. </p>
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<pre><strong>CREATE TABLE</strong> CORPDATA.DEPARTMENT
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(DEPTNO <strong>CHAR</strong>(3) <strong>NOT NULL PRIMARY KEY</strong>,
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DEPTNAME <strong>VARCHAR</strong>(29) <strong>NOT NULL</strong>,
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MGRNO <strong>CHAR</strong>(6),
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ADMRDEPT <strong>CHAR</strong>(3) <strong>NOT NULL</strong>
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<strong>CONSTRAINT</strong> REPORTS_TO_EXISTS
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<strong>REFERENCES</strong> CORPDATA.DEPARTMENT (DEPTNO)
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<strong>ON DELETE CASCADE</strong>)
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<strong>CREATE TABLE</strong> CORPDATA.EMPLOYEE
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(EMPNO <strong>CHAR</strong>(6) <strong>NOT NULL PRIMARY KEY</strong>,
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FIRSTNME <strong>VARCHAR</strong>(12) <strong>NOT NULL</strong>,
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MIDINIT <strong>CHAR</strong>(1) <strong>NOT NULL</strong>,
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LASTNAME <strong>VARCHAR</strong>(15) <strong>NOT NULL</strong>,
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WORKDEPT <strong>CHAR</strong>(3) <strong>CONSTRAINT</strong> WORKDEPT_EXISTS
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<strong>REFERENCES</strong> CORPDATA.DEPARTMENT (DEPTNO)
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<strong>ON DELETE SET NULL ON UPDATE RESTRICT</strong>,
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PHONENO <strong>CHAR</strong>(4),
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HIREDATE <strong>DATE</strong>,
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JOB <strong>CHAR</strong>(8),
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EDLEVEL <strong>SMALLINT NOT NULL,</strong>
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SEX <strong>CHAR</strong>(1),
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BIRTHDATE <strong>DATE</strong>,
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SALARY <strong>DECIMAL</strong>(9,2),
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BONUS <strong>DECIMAL</strong>(9,2),
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COMM <strong>DECIMAL</strong>(9,2),
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<strong>CONSTRAINT</strong> UNIQUE_LNAME_IN_DEPT <strong>UNIQUE</strong> (WORKDEPT, LASTNAME))</pre>
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</div>
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<div class="section"><p>In this case, the DEPARTMENT table has a column of unique department
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numbers (DEPTNO) which functions as a primary key, and is a parent table in
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two constraint relationships: </p>
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<dl><dt class="dlterm">REPORTS_TO_EXISTS</dt>
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<dd>is a self-referencing constraint in which the DEPARTMENT table is both
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the parent and the dependent in the same relationship. Every non-null value
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of ADMRDEPT must match a value of DEPTNO. A department must report to an existing
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department in the database. The DELETE CASCADE rule indicates that if a row
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with a DEPTNO value <em>n</em> is deleted, every row in the table for which
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the ADMRDEPT is <em>n</em> is also deleted.</dd>
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<dt class="dlterm">WORKDEPT_EXISTS</dt>
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<dd>establishes the EMPLOYEE table as a dependent table, and the column of
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employee department assignments (WORKDEPT) as a foreign key. Thus, every value
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of WORKDEPT must match a value of DEPTNO. The DELETE SET NULL rule says that
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if a row is deleted from DEPARTMENT in which the value of DEPTNO is <em>n</em>,
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then the value of WORKDEPT in EMPLOYEE is set to null in every row in which
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the value was <em>n</em>. The UPDATE RESTRICT rule says that a value of DEPTNO
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in DEPARTMENT cannot be updated if there are values of WORKDEPT in EMPLOYEE
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that match the current DEPTNO value.</dd>
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</dl>
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</div>
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<div class="section"><p>Constraint UNIQUE_LNAME_IN_DEPT in the EMPLOYEE table causes LASTNAME
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to be unique within a department. While this constraint is unlikely, it illustrates
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how a constraint made up of several columns can be defined at the table level.</p>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafyrefinteg.htm" title="Referential integrity is the condition of a set of tables in a database in which all references from one table to another are valid.">Referential integrity and tables</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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