blocks|key|166188|text|我会说是个错误的问题。DUAL与甲骨文有关，而不是MyBatis。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|166189|它是一个单行，单列表，由SYS拥有.看起来是这样的：|166190|SQL>+desc+dual
+Name++++++++++++++++++++++++++Null?++++Type
+-----------------------------+--------+--------------------
+DUMMY++++++++++++++++++++++++++++++++++VARCHAR2(1)

SQL>+select+*+from+dual;

DUMMY
-----
X

SQL>|code-block|syntax|javascript|166191|对于用户来说，它可以作为一个公共同义词：|166192|SQL>+select+object_name,+owner,+object_type
++2++from+all_objects
++3++where+object_name+=+'DUAL';

OBJECT_NAME++++++++++OWNER++++++++++++++++OBJECT_TYPE
--------------------+--------------------+-------------------
DUAL+++++++++++++++++SYS++++++++++++++++++TABLE
DUAL+++++++++++++++++PUBLIC+++++++++++++++SYNONYM

SQL>|166193|它是如何被创造出来的，为什么它有一个奇怪的名字，“双重”？|166194|查尔斯·维斯：|166195|我在Oracle数据字典中创建了作为底层对象的对偶表。它从来没有被看到自己，而是使用在一个视图，预计将被查询。这样做的想法是，您可以对双表进行连接，并在结果中为表中的每一行创建两行。然后，通过使用GROUP+BY，可以对结果的联接进行汇总，以显示数据范围和索引范围的存储量。这个名字，双行，似乎适合于从一个行创建一对行的过程。|blockquote|166196|我们在哪里使用它？无处不在！|BOLD|166197|在Oracle的SQL中，您必须从某些内容中进行选择，这通常是双重的。例如，今天是哪一天，或者我是谁？|166198|SQL>+select+sysdate+from+dual;

SYSDATE
-------------------
04.08.2022+11:43:08

SQL>+select+'My+name+is+Littlefoot'+who_am_i+from+dual;

WHO_AM_I
---------------------
My+name+is+Littlefoot

SQL>|166199|或者，在您的例子中，您选择序列的当前值。下面是一个演示：|166200|创建序列：|166201|SQL>+create+sequence+myseq;

Sequence+created.|166202|您的查询(但不起作用-请参阅错误描述)：|166203|SQL>+select+myseq.currval+from+dual;
select+myseq.currval+from+dual
+++++++*
ERROR+at+line+1:
ORA-08002:+sequence+MYSEQ.CURRVAL+is+not+yet+defined+in+this+session|166204|好的，让我们先去取nextval|166205|SQL>+select+myseq.nextval+from+dual;

+++NEXTVAL
----------
+++++++++1|166206|现在我们也有了currval|166207|SQL>+select+myseq.currval+from+dual;

+++CURRVAL
----------
+++++++++1

SQL>|166208|entityMap^0|B|4|0|0|0|0|0|0|0|0|9|5|0|0|0|0|0|0|0|0|9|7|0|0|7|7|0|0^^$0|@$1|2|3|4|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1R|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1S|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1T|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1U|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1V|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1W|8|@]|D|@]|E|$]]|$1|U|3|V|5|W|7|1X|8|@]|D|@]|E|$]]|$1|X|3|Y|5|6|7|1Y|8|@$9|1Z|A|20|B|Z]]|D|@]|E|$]]|$1|10|3|11|5|6|7|21|8|@]|D|@]|E|$]]|$1|12|3|13|5|J|7|22|8|@]|D|@]|E|$K|L]]|$1|14|3|15|5|6|7|23|8|@]|D|@]|E|$]]|$1|16|3|17|5|6|7|24|8|@]|D|@]|E|$]]|$1|18|3|19|5|J|7|25|8|@]|D|@]|E|$K|L]]|$1|1A|3|1B|5|6|7|26|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|J|7|27|8|@]|D|@]|E|$K|L]]|$1|1E|3|1F|5|6|7|28|8|@$9|29|A|2A|B|C]]|D|@]|E|$]]|$1|1G|3|1H|5|J|7|2B|8|@]|D|@]|E|$K|L]]|$1|1I|3|1J|5|6|7|2C|8|@$9|2D|A|2E|B|C]]|D|@]|E|$]]|$1|1K|3|1L|5|J|7|2F|8|@]|D|@]|E|$K|L]]|$1|1M|3|-4|5|6|7|2G|8|@]|D|@]|E|$]]]|1N|$]]

Wrong question, I'd say. <code>DUAL</code> is related to Oracle, not MyBatis.
It is a single-row, single-column table, owned by SYS. It looks like this:
<pre><code>SQL&gt; desc dual
 Name Null? Type
 ----------------------------- -------- --------------------
 DUMMY VARCHAR2(1)

SQL&gt; select * from dual;

DUMMY
-----
X

SQL&gt;
</code></pre>
To users, it is available as a public synonym:
<pre><code>SQL&gt; select object_name, owner, object_type
 2 from all_objects
 3 where object_name = 'DUAL';

OBJECT_NAME OWNER OBJECT_TYPE
-------------------- -------------------- -------------------
DUAL SYS TABLE
DUAL PUBLIC SYNONYM

SQL&gt;
</code></pre>
<hr />
How was it created and why does it have that strange name, &quot;dual&quot;?
Charles Weiss:
<blockquote>
I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage for the DATA extent and for the INDEX extent(s). The name, DUAL, seemed apt for the process of creating a pair of rows from just one.
</blockquote>
<hr />
Where do we use it? Everywhere!!!
In Oracle's SQL, you have to select from something, and that's frequently dual. For example, which date is it today, or who am I?
<pre><code>SQL&gt; select sysdate from dual;

SYSDATE
-------------------
04.08.2022 11:43:08

SQL&gt; select 'My name is Littlefoot' who_am_i from dual;

WHO_AM_I
---------------------
My name is Littlefoot

SQL&gt;
</code></pre>
<hr />
Or, in your case, you're selecting sequence's current value. Here's a demo:
Create the sequence:
<pre><code>SQL&gt; create sequence myseq;

Sequence created.
</code></pre>
Your query (won't work, though - see error description):
<pre><code>SQL&gt; select myseq.currval from dual;
select myseq.currval from dual
 *
ERROR at line 1:
ORA-08002: sequence MYSEQ.CURRVAL is not yet defined in this session
</code></pre>
OK, so let's first fetch <code>nextval</code>:
<pre><code>SQL&gt; select myseq.nextval from dual;

 NEXTVAL
----------
 1
</code></pre>
Now we also have the <code>currval</code>:
<pre><code>SQL&gt; select myseq.currval from dual;

 CURRVAL
----------
 1

SQL&gt;
</code></pre>

blocks|key|690847|text|从Oracle文档中|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|690848|DUAL是Oracle与数据字典一起自动创建的表。DUAL位于用户SYS的架构中，但所有用户都可以使用名称DUAL来访问。它有一个列DUMMY，定义为VARCHAR2(1)，并包含一个值为X的一行。从DUAL表中进行选择对于使用SELECT语句计算常量表达式非常有用。因为DUAL只有一行，所以只返回一次常量。|blockquote|style|CODE|690849|因此，该语句在您的会话中查询序列mySeq的当前值(因此必须在此之前调用nextval才能工作)。|690850|在Oracle中，您总是必须从某些内容中进行选择；如果没有实际的表要从中获取数据，那么dual是有用的--无论是对于常量还是文本或函数(select+sysadte+from+dual)等等。|690851|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/Selecting-from-the-DUAL-Table.html^0|0|A|0|0|0|4|P|4|X|3|1U|5|23|B|2M|1|2S|4|36|6|3S|4|0|G|5|10|7|0|17|4|1W|O|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@$A|Z|B|10|G|H]|$A|11|B|12|G|H]|$A|13|B|14|G|H]|$A|15|B|16|G|H]|$A|17|B|18|G|H]|$A|19|B|1A|G|H]|$A|1B|B|1C|G|H]|$A|1D|B|1E|G|H]|$A|1F|B|1G|G|H]]|9|@]|C|$]]|$1|I|3|J|5|6|7|1H|8|@$A|1I|B|1J|G|H]|$A|1K|B|1L|G|H]]|9|@]|C|$]]|$1|K|3|L|5|6|7|1M|8|@$A|1N|B|1O|G|H]|$A|1P|B|1Q|G|H]]|9|@]|C|$]]|$1|M|3|-4|5|6|7|1R|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

<a href="https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/Selecting-from-the-DUAL-Table.html" rel="nofollow noreferrer">From the Oracle documentation</a>:
<blockquote>
<code>DUAL</code> is a table automatically created by Oracle Database along with the data dictionary. <code>DUAL</code> is in the schema of the user <code>SYS</code> but is accessible by the name DUAL to all users. It has one column, <code>DUMMY</code>, defined to be <code>VARCHAR2(1)</code>, and contains one row with a value <code>X</code>. Selecting from the <code>DUAL</code> table is useful for computing a constant expression with the <code>SELECT</code> statement. Because <code>DUAL</code> has only one row, the constant is returned only once.
</blockquote>
So that statement is querying the current value of the sequence <code>mySeq</code>, within your session (so there must have been a call to <code>nextval</code> preceding it for it to work).
In Oracle SQL you always have to select from something; <code>dual</code> is useful when there is no actual table you want to get data from - either for a constant or literal or function (<code>select sysadte from dual</code>) etc.

I am just trying to understand some old code without help which uses this mybatis statement. I did not find any proper answer. So in the below statement of myBatis what is dual?
<pre><code>SELECT mySeq.currVal FROM dual
</code></pre>

What is dual in mybatis?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我只是试图理解一些旧的代码，没有帮助，它使用这条马提斯语句。我没有找到合适的答案。那么，在下面的myBatis声明中，什么是双重的？SELECT mySeq.currVal FROM dual

问什么是在紫花莲的双重？
EN

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问什么是在紫花莲的双重？EN