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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>Select Statement</title> <link rel="StyleSheet" href="css/dbasfox.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="dbasfox.21.01.html#106947">16 The dBASE Driver</a> : Select Statement</div> <hr align="left" /> <blockquote> <div class="head_1"><a name="106947">Select Statement</a></div> <div class="Body"><a name="106948">You use a SQL Select statement to specify the columns and records to be read. All of the </a>Select statement clauses described in <span class="Cross_ref_"><a href="javascript:WWHClickedPopup('reference', 'flatfile.13.1.html#106145', '');">Chapter 11 â??SQL Statements for Flat-File Driversâ??</a></span> in the <span class="Emphasis">DataDirect Connect Series </span><span class="forbody">for</span> <span class="Emphasis" style="font-size: 8.0pt;">ODBC</span><span class="Emphasis" style="font-size: 10.0pt;"> Reference</span> are supported by dBASE Select statements. This section describes the information that is specific to dBASE, which is Rowid.</div> <div class="head_2_no_TOC"><a name="106960">Rowid Pseudo-Column</a></div> <div class="Body"><a name="106961">Each dBASE record contains a special column named Rowid. This field contains a unique </a>number that indicates the recordâ??s sequence in the database. For example, a table that contains 50 records has Rowid values from 1 to 50 (if no records are marked deleted). You can use Rowid in Where and Select clauses.</div> <div class="Body"><a name="106962">Rowid is particularly useful when you are updating records. You can retrieve the Rowid of </a>the records in the database along with the other field values. For example:</div> <div class="syntax_first"><a name="106963">SELECT last_name, first_name, salary, rowid FROM emp</a></div> <div class="Body"><a name="106964">Then, you can use the Rowid of the record that you want to update to ensure that you are </a>updating the correct record and no other. For example:</div> <div class="syntax_first"><a name="106965">UPDATE emp set salary = 40000 FROM emp WHERE rowid=21</a></div> <div class="Body"><a name="106966">The fastest way of updating a single row is to use a Where clause with the Rowid. You </a>cannot update the Rowid column.</div> <div class="Body"><a name="106967">Select statements that use the Rowid pseudo-column in the Where clause achieve </a>maximum performance only for exact equality matches. If you use range scans instead of exact equality matches, a full table scan is performed. For example:</div> <div class="syntax_first"><a name="106968">SELECT * FROM emp WHERE rowid=21</a> //fast search</div> <div class="syntax"><a name="106969">SELECT * FROM emp WHERE rowid <=25</a> //full table scan</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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