Dual tables in Oracle

1. Strange, no specific table structure information found

Sql> describe dual;
Name Type Nullable Default Comments
----- ----------- -------- ------- --------
DUMMY VARCHAR2 (1) Y

Sql> SELECT * from dual;
DUMMY
-----
X

Purpose of the 2.DUAL table
Dual is an actual table in Oracle that can be read by any user, often in a SELECT statement block without a target table.

To query the current user:
Sql> Select User from dual;
USER
------------------------------
Test_user

Query system Date Time

Sql> select Sysdate from dual;
Sysdate
-----------
2015/10/15

To achieve a simple calculation:

Sql> select 12*3 from dual;

12*3
----------
36
Only allow querying data from the dual table, you cannot update,insert,delete, the database does not start properly after you delete the table.

Dual tables in Oracle