1Z0-51 QUESTION 18 DISTINCT的用法

QUESTION 18

Examine the structure of the PROMOTIONS table:

name Null Type
PROMO_ID NOT NULL NUMBER(6)
PROMO_NAME NOT NULL VARCHAR2(30)
PROMO_CATEGORY NOT NULL VARCHAR2(30)
PROMO_COST NOT NULL NUMBER(10,2)
The management wants to see a report of unique promotion costs in each promotion category.
Which query would achieve the required result?
A. SELECT DISTINCT promo_cost, promo_category FROM promotions;
B. SELECT promo_category, DISTINCT promo_cost FROM promotions;
C. SELECT DISTINCT promo_cost, DISTINCT promo_category FROM promotions;

D. SELECT DISTINCT promo_category, promo_cost FROM promotions ORDER BY 1;

答案:D

解析：

--A选项能够得出结果，但不合题意，这个是每个促销价格都有什么种类
SQL> SELECT DISTINCT promo_cost, promo_category FROM promotions;

PROMO_COST PROMO_CATEGORY
---------- ------------------------------------------------------------
     30000 promocategory2
     20000 promocategory1
     40000 promocategory3
     30000 promocategory3
     30000 promocategory5
--B选项报错
SQL> SELECT promo_category, DISTINCT promo_cost FROM promotions;
SELECT promo_category, DISTINCT promo_cost FROM promotions
                       *
ERROR at line 1:
ORA-00936: missing expression

--C选项报错
SQL> SELECT DISTINCT promo_cost, DISTINCT promo_category FROM promotions;
SELECT DISTINCT promo_cost, DISTINCT promo_category FROM promotions
                            *
ERROR at line 1:
ORA-00936: missing expression

--D选项得出正确的答案。
SQL> SELECT DISTINCT promo_category, promo_cost FROM promotions ORDER BY 1;

PROMO_CATEGORY                                               PROMO_COST
------------------------------------------------------------ ----------
promocategory1                                                    20000
promocategory2                                                    30000
promocategory3                                                    30000
promocategory3                                                    40000
promocategory5                                                    30000