The role of the dual table for Oracle databases

Dual is the smallest worksheet provided by Oracle, with a single row and some special functions.

Regardless of what you do (don't delete records), it has only one record-' X '.

Role:

1. Select evaluates constant expression and pseudo-column equivalence. The dual table can only return one piece of data, and multiple data may be returned when using a different table.

2. View Current User

Select User from Dual;

Select COUNT (*) from dual;

3. As a calculator

Select 7*9*10-10 from Fual;

4. Call system functions

Get current system time

Select To_char (sysdate, ' Yyyy-mm-dd hh24:mi:ss ') from dual;

Get host Name

Select Sys_context (' Userenv ', ' terminal ') from dual;

Get current locale

Select Sys_context (' Userenv ', ' language ') from dual;

Get a random number

Select Dbms_random.random from dual;

5. View sequence values

Create sequence AAA starting at 1, plus 1 per time

Create sequence AAA increment by 1 start with 1;

Gets the next sequence value of the sequence AAA

Select Aaa.nextval from dual;

Gets the current sequence value of the sequence AAA

Select Aaa.currval from dual;

The role of the dual table for Oracle databases