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DUAL table

The following example returns the current operating system date and time

Section titled “The following example returns the current operating system date and time”
select sysdate from dual

The following example generates numbers between start_value and end_value

Section titled “The following example generates numbers between start_value and end_value”
select :start_value + level -1 n
from dual
connect by level <= :end_value  - :start_value + 1

Remarks

Section titled “Remarks”

DUAL table has one column DUMMY, defined to be VARCHAR2(1) and only one row with a value x.

DUAL table is automatically created in SYS schema when database is created. You can access it from any schema.

You can not change DUAL table.

You can use DUAL table to call any function from SQL statement. It is useful because it has only one row and oracle optimizer knows everything about it.