Pseudocolumn
A Pseudocolumn is a "column" that yields a value when selected, but which is not an actual column of the table. An example is RowID or SysDate. It is often used in combination with the DUAL table The DUAL table is a special one-row, one-column table present by default in Oracle and other database installations. In Oracle, the table has a single VARCHAR2(1) column called DUMMY that has a value of 'X'. It is suitable for use in selecting a pse .... References Databases {{Database-stub ...
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DUAL Table
The DUAL table is a special one-row, one-column table present by default in Oracle and other database installations. In Oracle, the table has a single VARCHAR2(1) column called DUMMY that has a value of 'X'. It is suitable for use in selecting a pseudo column such as SYSDATE or USER. Example use Oracle's SQL syntax requires the FROM clause but some queries don't require any tables - DUAL can be used in these cases. SELECT 1+1 FROM dual; SELECT 1 FROM dual; SELECT USER FROM dual; SELECT SYSDATE FROM dual; SELECT * FROM dual; History Charles Weiss explains why he created DUAL: I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage ...
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