于是以“X$DUAL oracle”为关键词google了一下,找到Tom的一篇文章 1.Whatisthe dual table, whatisits purpose. dualisjust a convienence table. You don't need to use it, you can use anything youwant. The advantagetodualisthe optimizer understands dualisa special one row, one column table --when...
column table-- when you use it in queries, it uses this knowledge when developing theplan.2.Whydoesitcontainonly one columnwithdatatype varchar2, whynotnumber. truly, why no. Whynotadateyou would askthen. The column,itsname,itsdatatypeandevenitsvalue are NOT relevant. DUAL exists solelyasa ...
In Oracle, theSELECTstatement must have aFROMclause. However, some queries don’t require any table for example: SELECTUPPER('This is a string')FROMwhat_table;Code language:SQL (Structured Query Language)(sql) In this case, you might think about creating a table and use it in theFROMclaus...
首先,公司用的是Oracle数据库,关于Oracle数据库中的dual表,官方文档说明(The DUAL Table): DUALis a small table in the data dictionary that Oracle Database and user-written programs can reference to guarantee a known result. The dual table is useful when a value must be returned only once, for ...
dual本身就是一个单词,并非是某个词组的缩写.名词意为对数,形容词为双重的意思.在Oracle中dual指dual table,一个单行单列的虚拟表.1: DUAL table-- a special kind of one-row and one-column database table;2:dual is a table which is created by oracle along with the data dictionary. ...
266 What is the dual table in Oracle? 1 Recreate table with data and relations 6 The magic of DUAL 5 Oracle select from dual with multiple rows and columns 1 How to get this below output from DUAL in oracle? 2 Internal Functionality of DUAL table? 0 how to create a view in orac...
根据维基百科,DUAL表是由Oracle工作人员Chuck Weiss创建的。它最初用于连接内部视图: 我在Oracle数据字典中创建了DUAL表作为底层对象。它本来就不应该被自己看到,而是在一个需要查询的视图中使用。其想法是,您可以对DUAL表执行JOIN,并在结果中为表中的每一行创建两行。然后,通过使用GROUP by,可以对生成的连接进行汇总...
The table named DUAL is a small table in the data dictionary that Oracle and user-written programs can reference to guarantee a known result. This table has one column called DUMMY and one row containing the value X. 这个名称为dual的表是位了保证oracle和用户程序能够引用一个已知的值。这个表只...
So in reality, because the name DUAL is such a misnomer, if I were to create a table and name it ATOM or ONE, e.g. create table one (atom int); .. select 'abc' abc, 1 def FROM one; - Is there a performance penalty compared to SELECT .. FROM DUAL? oracle select Share Improv...
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,iVBORw0KGgoAAAANSUhEUgAAAL4AAABkCAIAAAC3sx2D...