Dual表是每个数据库创建时默认生成的,该表仅有一列一行。

1)分析dual表执行,如下:

aaarticlea/png;base64,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" alt="" />

2)应用:

在oracle数据库中,dual表作为一个虚表存在的,因为oracle的查询操作语句必须满足格式:select columnname from tablename,其中的from所指向的表示必须的,所以某些非查询操作可以通过dual表来实现,如:

--查看当前日期:

Select sysdate from dual ;

aaarticlea/png;base64,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" alt="" />

--计算等式:

Select 3*3 – 3 as result, 8-9 as plus from dual;

aaarticlea/png;base64,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" alt="" />

--拼接字符串:

Select 'this' ||'is'||'test' from dual;

aaarticlea/png;base64,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" alt="" />

--执行其他无特定表操作

如系统函数等。

---------

本系列为最近一段时间学习oracle的学习笔记,记录于此作为自身回顾,其中有的来的网络,有的来的书籍,但时间已久,记不清哪些是引用,如是转载但没标注出,特意致歉。

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