📄 oracle sql性能优化系列 (三)--acnow_net.htm
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<H1>ORACLE SQL性能优化系列 (三)</H1>
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<TD align=middle height=20>http://Tech.acnow.net 2005-4-29 <FONT color=#a20010>网络</FONT></TD>
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<TD class=l17><FONT class=f14 id=zoom><BR> <BR><BR>8. 使用DECODE函数来减少处理时间<BR><BR> <BR><BR>使用DECODE函数可以避免重复扫描相同记录或重复连接相同的表.<BR><BR> <BR><BR>例如:<BR><BR> SELECT COUNT(*)，SUM(SAL)<BR><BR> FROM EMP<BR><BR> WHERE DEPT_NO = 0020<BR><BR> AND ENAME LIKE ‘SMITH%’;<BR><BR> <BR><BR> SELECT COUNT(*)，SUM(SAL)<BR><BR> FROM EMP<BR><BR> WHERE DEPT_NO = 0030<BR><BR> AND ENAME LIKE ‘SMITH%’;<BR><BR> <BR><BR>你可以用DECODE函数高效地得到相同结果<BR><BR> <BR><BR>SELECT COUNT(DECODE(DEPT_NO,0020,’X’,NULL)) D0020_COUNT,<BR><BR> COUNT(DECODE(DEPT_NO,0030,’X’,NULL)) D0030_COUNT,<BR><BR> SUM(DECODE(DEPT_NO,0020,SAL,NULL)) D0020_SAL,<BR><BR> SUM(DECODE(DEPT_NO,0030,SAL,NULL)) D0030_SAL<BR><BR>FROM EMP WHERE ENAME LIKE ‘SMITH%’;<BR><BR> <BR><BR>类似的,DECODE函数也可以运用于GROUP BY 和ORDER BY子句中.<BR><BR> <BR><BR> <BR><p><div align="right">本新闻共<font color=red>2</font>页,当前在第<font color=red>1</font>页 <font color="red">1</font> <a href="164558059816455894796_2.shtml">2</a> </div></p>
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<td><img src="http://tech.acnow.net/Files/Img/list_nav_spacer.gif"><a Class="a2" href= http://tech.acnow.net/Html/DataBase/Oracle/2005-4/29/164559059816455960820.shtml title="ORACLE SQL性能优化系列 (十四) 完结篇">ORACLE SQL性能优化系列 (十四) 完结篇</a></td></tr>
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<td><img src="http://tech.acnow.net/Files/Img/list_nav_spacer.gif"><a Class="a2" href= http://tech.acnow.net/Html/DataBase/Oracle/2005-4/29/164559059816455944859.shtml title="ORACLE SQL性能优化系列 (十)">ORACLE SQL性能优化系列 (十)</a></td></tr>
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