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<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xml:lang="en" lang="en" xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <!-- MOTW-DISABLED saved from url=(0014)about:internet --> <title>Indexing Multiple Fields</title> <link rel="StyleSheet" href="css/useindx.css" type="text/css" media="all" /> <link rel="StyleSheet" href="css/webworks.css" type="text/css" media="all" /> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/context.js"></script> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/towwhdir.js"></script> <script type="text/javascript" language="JavaScript1.2" src="wwhdata/common/wwhpagef.js"></script> <script type="text/javascript" language="JavaScript1.2"> <!-- var WebWorksRootPath = ""; // --> </script> <script type="text/javascript" language="JavaScript1.2"> <!-- // Set reference to top level help frame // var WWHFrame = WWHGetWWHFrame("", true); // --> </script> <script type="text/javascript" language="JavaScript1.2" src="scripts/expand.js"></script> </head> <body class="" style="background-color: #FFFFEE;" onload="WWHUpdate();" onunload="WWHUnload();" onkeydown="WWHHandleKeyDown((document.all||document.getElementById||document.layers)?event:null);" onkeypress="WWHHandleKeyPress((document.all||document.getElementById||document.layers)?event:null);" onkeyup="WWHHandleKeyUp((document.all||document.getElementById||document.layers)?event:null);"> <br /> <div class="WebWorks_Breadcrumbs" style="text-align: left;"> <a class="WebWorks_Breadcrumb_Link" href="useindx.08.1.html#106198">6 Using Indexes</a> : Indexing Multiple Fields</div> <hr align="left" /> <blockquote> <div class="head_1"><a name="106198">Indexing Multiple Fields</a></div> <div class="Body"><a name="106199">If you often use Where clauses that involve more than one field, you may want to build an </a>index containing multiple fields. Consider the following Where clause:</div> <div class="syntax_first"><a name="106200">WHERE last_name = 'Smith' AND first_name = 'Thomas'</a></div> <div class="Body"><a name="106201">For this condition, the optimal index field expression is last_name, first_name. This creates </a>a concatenated index.</div> <div class="Body"><a name="106202">Concatenated indexes can also be used for Where clauses that contain only the first of two </a>concatenated fields. The last_name, first_name index also improves the performance of the following Where clause (even though no first name value is specified):</div> <div class="syntax_first"><a name="106203">last_name = 'Smith'</a></div> <div class="Body"><a name="106204">Consider the following Where clause:</a></div> <div class="syntax_first"><a name="106205">WHERE last_name = 'Smith' AND middle_name = 'Edward' AND first_name = 'Thomas'</a></div> <div class="Body"><a name="106206">If your index fields include all the conditions of the Where clause in that order, the driver can </a>use the entire index. If, however, your index is on two nonconsecutive fields, for example, last_name and first_name, the driver can use only the last_name field of the index.</div> <div class="Body"><a name="106207">The driver uses only one index when processing Where clauses. If you have complex </a>Where clauses that involve a number of conditions for different fields and have indexes on more than one field, the driver chooses an index to use. The driver attempts to use indexes on conditions that use the equal sign as the relational operator rather than conditions using other operators (such as greater than). Assume you have an index on the emp_id field as well as the last_name field and the following Where clause:</div> <div class="syntax_first"><a name="106208">WHERE emp_id >= 'E10001' AND last_name = 'Smith'</a></div> <div class="Body"><a name="106209">In this case, the driver selects the index on the last_name field.</a></div> <div class="Body"><a name="106210">If no conditions have the equal sign, the driver first attempts to use an index on a condition </a>that has a lower <span class="Emphasis">and</span> upper bound, and then attempts to use an index on a condition that has a lower <span class="Emphasis">or</span> upper bound. The driver always attempts to use the most restrictive index that satisfies the Where clause.</div> <div class="Body"><a name="106211">In most cases, the driver does not use an index if the Where clause contains an OR </a>comparison operator. For example, the driver does not use an index for the following Where clause:</div> <div class="syntax_first"><a name="106212">WHERE emp_id >= 'E10001' OR last_name = 'Smith'</a></div> <script type="text/javascript" language="JavaScript1.2"> <!-- // Clear related topics // WWHClearRelatedTopics(); document.writeln(WWHRelatedTopicsInlineHTML()); // --> </script> </blockquote> <script type="text/javascript" language="JavaScript1.2"> <!-- document.write(WWHRelatedTopicsDivTag() + WWHPopupDivTag() + WWHALinksDivTag()); // --> </script> </body> </html>
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