163 lines
7.7 KiB
HTML
163 lines
7.7 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<meta name="DC.Title" content="Handle null values" />
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<meta name="abstract" content="A NULL value indicates the absence of a column value in a row." />
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<meta name="description" content="A NULL value indicates the absence of a column value in a row." />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Handle null values</title>
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</head>
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<body id="rbafynulls"><a name="rbafynulls"><!-- --></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">Handle null values</h1>
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<div><p>A <em>NULL</em> value indicates the absence of a column value in
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a row.</p>
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<div class="section"><p>A null value is not the same as zero or all blanks. A null value
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means unknown. Null values can be used as a condition in the WHERE and HAVING
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clauses. For example, a WHERE clause can specify a column that, for some rows,
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contains a null value. A basic comparison predicate using a column that contains
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null values does not select a row that has a null value for the column. This
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is because a null value is neither less than, equal to, nor greater than the
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value specified in the condition. The IS NULL predicate is used to check for
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null values. To select the values for all rows that contain a
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null value for the manager number, you can specify:</p>
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</div>
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<div class="section"><pre><strong>SELECT</strong> DEPTNO, DEPTNAME, ADMRDEPT
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<strong>FROM</strong> CORPDATA.DEPARTMENT
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<strong>WHERE</strong> MGRNO <strong>IS NULL</strong></pre>
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<p>The result are:</p>
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</div>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" width="100%" frame="hsides" border="1" rules="all"><thead align="left"><tr><th align="left" valign="bottom" id="d0e56">DEPTNO</th>
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<th align="left" valign="bottom" id="d0e58">DEPTNAME</th>
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<th align="left" valign="bottom" id="d0e60">ADMRDEPT</th>
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</tr>
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</thead>
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<tbody><tr><td align="left" valign="top" headers="d0e56 ">D01</td>
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<td align="left" valign="top" headers="d0e58 ">DEVELOPMENT CENTER</td>
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<td align="left" valign="top" headers="d0e60 ">A00</td>
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</tr>
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<tr><td valign="top" headers="d0e56 ">F22</td>
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<td valign="top" headers="d0e58 ">BRANCH OFFICE F2</td>
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<td valign="top" headers="d0e60 ">E01</td>
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</tr>
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<tr><td valign="top" headers="d0e56 ">G22</td>
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<td valign="top" headers="d0e58 ">BRANCH OFFICE G2</td>
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<td valign="top" headers="d0e60 ">E01</td>
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</tr>
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<tr><td valign="top" headers="d0e56 ">H22</td>
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<td valign="top" headers="d0e58 ">BRANCH OFFICE H2</td>
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<td valign="top" headers="d0e60 ">E01</td>
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</tr>
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<tr><td valign="top" headers="d0e56 ">I22</td>
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<td valign="top" headers="d0e58 ">BRANCH OFFICE I2</td>
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<td valign="top" headers="d0e60 ">E01</td>
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</tr>
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<tr><td valign="top" headers="d0e56 ">J22</td>
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<td valign="top" headers="d0e58 ">BRANCH OFFICE J2</td>
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<td valign="top" headers="d0e60 ">E01</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section"><p>To get the rows that do not have a null value for the manager
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number, you can change the WHERE clause like this:</p>
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</div>
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<div class="section"><pre><strong>WHERE</strong> MGRNO <strong>IS NOT NULL</strong></pre>
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</div>
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<div class="section"><p>Another predicate that is useful for comparing values that can
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contain the NULL value is the DISTINCT predicate. Comparing two columns using
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a normal equal comparison (COL1 = COL2) will be true if both columns contain
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an equal non-null value. If both columns are null, the result will be false
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since null is never equal to any other value, not even another null value.
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Using the DISTINCT predicate, null values are considered equal. So (COL1 is
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NOT DISTINCT from COL2) will be true if both columns contain an equal non-null
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value and also when both columns are the null value.</p>
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</div>
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<div class="section"><p>For example, suppose you want to select information from 2 tables
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that contain null values. The first table (T1) has a column (C1) with the
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following values: </p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="hsides" border="1" rules="all"><thead align="left"><tr valign="top"><th valign="top" id="d0e126">C1</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e126 ">2</td>
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</tr>
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<tr><td valign="top" headers="d0e126 ">1</td>
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</tr>
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<tr><td valign="top" headers="d0e126 ">null</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><p>The second table (T2) has a column (C2) with the following values: </p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="hsides" border="1" rules="all"><thead align="left"><tr valign="top"><th valign="top" id="d0e146">C2</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" headers="d0e146 ">2</td>
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</tr>
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<tr><td valign="top" headers="d0e146 ">null</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><p>Run the following SELECT statement: </p>
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<pre><strong>SELECT *
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FROM </strong>T1, T2
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<strong>WHERE </strong>C1 <strong>IS DISTINCT FROM</strong> C2</pre>
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<p>The results are:</p>
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<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" frame="hsides" border="1" rules="all"><thead align="left"><tr valign="top"><th valign="top" width="50%" id="d0e176">C1</th>
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<th valign="top" width="50%" id="d0e178">C2</th>
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</tr>
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</thead>
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<tbody><tr><td valign="top" width="50%" headers="d0e176 ">1</td>
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<td valign="top" width="50%" headers="d0e178 ">2</td>
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</tr>
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<tr><td valign="top" width="50%" headers="d0e176 ">1</td>
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<td valign="top" width="50%" headers="d0e178 ">-</td>
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</tr>
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<tr><td valign="top" width="50%" headers="d0e176 ">2</td>
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<td valign="top" width="50%" headers="d0e178 ">-</td>
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</tr>
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<tr><td valign="top" width="50%" headers="d0e176 ">-</td>
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<td valign="top" width="50%" headers="d0e178 ">2</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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<div class="section"><p>For more information about the use of null values, see the <a href="../db2/rbafzmst02.htm">SQL Reference</a> topic
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collection.</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="rbafytexas.htm" title="Learn a variety of ways of tailoring your query to gather data using the SELECT statement. One way to do this is to use the SELECT statement in a program to retrieve a specific row (for example, the row for an employee). Furthermore, you can use clauses to gather data in a specific way.">Retrieve data using the SELECT statement</a></div>
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</div>
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</div>
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</body>
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</html> |