循序渐进学Oracle之复杂查询(重点)
前言在实际应用中经常需要执行复杂的数据统计,经常需要显示多张表的数据,故必须要高度重视:复杂的select语句!1-数据分组的重要函数:max、min、avg、sum、count(1
前言
在实际应用中经常需要执行复杂的数据统计,经常需要显示多张表的数据,故必须要
范例：
没有40部门，我们换成RIGHT JOIN…ON ，如下图所示：
由上可知：40部门出现了！
在Oracle之外的数据库（SQLSERVER、MYSQL、DB2等）都使用以上的SQL：1999语法操作，故必须要会！（当然，，一直使用Oracle数据库，就可以不会：））
再次强调：多表查询的性能肯定不高，而且性能一定要在大数据量的情况下，才能发现。
本文出自 “beyondhedefang” 博客，请务必保留此出处
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