What is dual in Oracle?

1. Dual is indeed a table. is a table that has only one field and a single row of records .
2. In practice, we call it ' pseudo-table '. Because he doesn't store subject data.
3. His presence is for the convenience of operation. Because select is intended to have a specific object.
such as: SELECT * from MyTable;
select * from MyView;
and so on.
But if we do not need to get the table data from the specific table, but simply to get some information we want, and to complete through select, we have to use an object, this object is dual;
if we want to calculate the value of 999*999, we can use:
select 999*999 from dual;
to achieve;
to splice a phone message:
Select Concat (' 010-', ' 88888888 ') | | ' Ext. 23 ' Gao Hu call from dual;

it becomes the format output that we want.

4. Of course, we do not have to dual, or we can do so. For example:
CREATE TABLE mydual (dummy varchar2 (1));
you can also achieve and dual the same effect:
select 999*999 from mydual;
However, dual we are accustomed to, it is unnecessary to do a set of their own.