What is a dual table

milen
11:09/10.03.2008
2.4 from possible 5 with 7 votes

DUAL is a table automatically created by Oracle and accessible to all users. It has one column, DUMMY, containing one row. It’s useful for selecting a constant or expression, because the table always exists (so the query will succeed) and the result will only be returned once (since there’s exactly one row in the table).

For example, you can use it for math:
SELECT (319/212)+10 FROM DUAL;
You can use it to increment sequences:
SELECT employee_seq.NEXTVAL FROM DUAL;
You can use it to play around or test some SQL:
SELECT CHR(70) FROM DUAL;

It was named "DUAL" because the primary intention of this table was to allow users to create 2 rows for every row in a table by joining it to this system table. Nowadays it only has one row.