2.2 DUAL Table

DUAL is a table automatically created by Oracle Database along with the data dictionary.
DUAL is in the schema of the user SYS but is accessible by the name DUAL to all users.
It has one column, “DUMMY”, defined as VARCHAR2(1), and contains one row with a value “X”. Selecting from the DUAL table is useful for computing a constant expression with the SELECT statement.

SELECT 1 + 2 as res FROM DUAL;

SELECT 2*2/2 + 2*2 - 2 as res FROM DUAL;

SELECT 2*2/(2+2)*2 - 2 as res FROM DUAL;

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