76 lines
4.6 KiB
HTML
76 lines
4.6 KiB
HTML
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<meta name="abstract" content="A check constraint assures the validity of data during insert and update operations by limiting the allowable values in a column or group of columns." />
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<title>Add and use check constraints</title>
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<body id="rbafycheckcon"><a name="rbafycheckcon"><!-- --></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 and use check constraints</h1>
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<div><p>A <em>check constraint</em> assures the validity of data during insert
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and update operations by limiting the allowable values in a column
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or group of columns.</p>
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<div class="section"><p>Use the SQL CREATE TABLE and ALTER TABLE statements to add or
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drop check constraints.</p>
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</div>
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<div class="section"><p>In this example, the following statement creates a table with
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three columns and a check constraint over COL2 that limits the values allowed
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in that column to positive integers: </p>
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<pre><strong>CREATE TABLE</strong> T1 (COL1 <strong>INT</strong>, COL2 <strong>INT CHECK</strong> (COL2>0), COL3 <strong>INT</strong>)</pre>
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</div>
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<div class="section"><p>Given this table, the following statement:</p>
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<pre><strong>INSERT INTO</strong> T1 <strong>VALUES</strong> (-1, -1, -1)</pre>
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<p>fails
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because the value to be inserted into COL2 does not meet the check constraint;
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that is, -1 is not greater than 0.</p>
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</div>
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<div class="section"><p>The following statement is successful:</p>
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</div>
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<div class="example"> <pre><strong>INSERT INTO</strong> T1 <strong>VALUES</strong> (1, 1, 1)</pre>
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</div>
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<div class="section"><p>Once that row is inserted, the following statement fails: </p>
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<pre><strong>ALTER TABLE</strong> T1 <strong>ADD CONSTRAINT</strong> C1 <strong>CHECK</strong> (COL1=1 <strong>AND</strong> COL1<COL2)</pre>
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</div>
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<div class="section"><p>This ALTER TABLE statement attempts to add a second check constraint
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that limits the value allowed in COL1 to 1 and also effectively rules that
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values in COL2 be greater than 1. This constraint is not allowed because the
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second part of the constraint is not met by the existing data (the value of
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'1' in COL2 is not less than the value of '1' in COL1).</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="rbafymovnow.htm" title="DB2 UDB for iSeries supports unique, referential, and check constraints.">Constraints</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/rbafzmstatabl.htm">ALTER TABLE statement</a></div>
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<div><a href="../db2/rbafzmsthctabl.htm">CREATE TABLE statement</a></div>
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</div>
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