Dual
From Oracle FAQ
DUAL is a table owned by the SYS user that contains one row with value 'X'. This is handy when you want to select an expression and only want to get a single row back.
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Sample usage
Get current date:
SELECT sysdate FROM dual;
Do some maths:
SELECT 7*8/9 FROM dual;
Extract the next value from a sequence:
SELECT myseq.NEXTVAL FROM dual;
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History
According to legend this table originally contained two rows, from there the name DUAL.
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