FROM <<Oracle.Database.11g.SQL>>
dual is a table that contains a single row. The following output from the DESCRIBE command shows the structure of the dual table, along with a query that retrieves the row from the dual table:
DESCRIBE dual Name Null? Type ----------------------------------------- -------- ----------- DUMMY VARCHAR2(1)
SELECT * FROM dual; D - X
Notice the dual table has one VARCHAR2 column named dummy and contains a single row with the value X.
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SELECT TO_DATE(‘02-AUG-2007‘) - 3 FROM dual; TO_DATE(‘ --------- 30-JUL-07 You can also subtract one date from another, yielding the number of days between the two dates. The following example subtracts July 25, 2007, from August 2, 2007: SELECT TO_DATE(‘02-AUG-2007‘) - TO_DATE(‘25-JUL-2007‘) FROM dual; TO_DATE(‘02-AUG-2007‘)-TO_DATE(‘25-JUL-2007‘) --------------------------------------------- 8
NOTE:
TO_DATE() is a function that converts a string to a date.
SELECT 10 * 12 / 3 – 1 FROM dual; 10*12/3-1 ---------- 39
SELECT TO_DATE(‘02-AUG-2007‘) - 3 FROM dual; TO_DATE(‘ --------- 30-JUL-07
原文：http://www.cnblogs.com/mylinux/p/4991372.html