28.choose the best answer
Evaluate the following SQL statement:
SQL> SELECT promo_id, promo_category
FROM promotions
WHERE promo_category = ‘Internet‘ ORDER BY 2 DESC
UNION
SELECT promo_id, promo_category
FROM promotions
WHERE promo_category = ‘TV‘
UNION
SELECT promo_id, promo_category
FROM promotions
WHERE promo_category =‘Radio‘;
Which statement is true regarding the outcome of the above query?
A) It produces an error because the ORDER BY clause should appear only at the end of a compound query-that is, with the last SELECT statement.
B) It executes successfully and displays rows in the descending order of PROMO_CATEGORY.
C) It produces an error because positional notation cannot be used in the ORDER BY clause with SET operators.
D) It executes successfully but ignores the ORDER BY clause because it is not located at the end of the compound statement.
Answer：A
(执行时 order by 子句只能出现最后的 select 语句，否则出现“ORA-00933: SQL 命令未正确结束”错误)
原文地址：http://blog.51cto.com/13854012/2352847