DUAL
DUAL is a single-column "dummy" table with one record whose value is X; for example:
=> SELECT * FROM DUAL; dummy ------- X (1 row)
You can write the following types of queries:
=> SELECT 1 FROM dual; ?column? ---------- 1 (1 row) => SELECT current_timestamp, current_user FROM dual; ?column? | current_user -------------------------------+-------------- 2010-03-08 12:57:32.065841-05 | release (1 row) => CREATE TABLE t1(col1 VARCHAR(20), col2 VARCHAR(2)); => INSERT INTO T1(SELECT 'hello' AS col1, 1 AS col2 FROM dual);) => SELECT * FROM t1; col1 | col2 -------+------ hello | 1 (1 row
Restrictions
You cannot create projections for DUAL.