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Asked by Vijay Last Modified
Answer 
Within the realm of Oracle databases, the so-called DUAL table has its own special uses. To understand the full set of meanings, the table can be described in terms of the following features:
Purpose:
* The DUAL table is used primarily when selecting pseudo-columns or for performing calculations when there is no reference to data from any particular table.
* The kind of SQL statement that requires a table reference but does not actually retrieve any data from a valid data table can be worked out conveniently with this DUAL table.
Characteristics:
* It has only a single row and a single column.
* It has one column, namely, the DUMMY column, with a VARCHAR2(1) datatype and a value of 'X'.
* It is automatically created by Oracle and owned by the SYS schema, but accessible to all users.
Common Uses:
* Retrieving system values can be done by means of DUAL. Examples include obtaining the system date and time (SYSDATE), the current user (USER), or any other system-related information.
* Performing calculations: The DUAL table can become useful for simple arithmetic or expression evaluation.
* Verification of the SQL syntax: A handy way to test out SQL statement syntax.
Hence, the DUAL table is a service table whose main utility is for executing SQL statements that do not receive an input from user-defined tables.
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