OCP 1Z0 051 18

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; 

该题中A、D都能执行。但A显示顺序不对

SQL> SELECT *
  2    FROM (SELECT DISTINCT promo_cost, promo_category FROM sh.promotions)
  3   WHERE rownum <= 10;
  PROMO_COST PROMO_CATEGORY
------------ ------------------------------
        0.00 ad news
        0.00 NO PROMOTION
      200.00 newspaper
      300.00 post
      400.00 newspaper
      600.00 internet
     1100.00 TV
     1400.00 internet
     1500.00 TV
     2000.00 post
10 rows selected


SELECT promo_category, DISTINCT promo_cost FROM sh.promotions
ORA-00936: 缺失表达式

SELECT DISTINCT promo_cost, DISTINCT promo_category FROM sh.promotions
ORA-00936: 缺失表达式

SQL> SELECT * 2 FROM (SELECT DISTINCT promo_category, promo_cost 3 FROM sh.promotions 4 ORDER BY 1) 5 WHERE rownum <= 10;PROMO_CATEGORY PROMO_COST------------------------------ ------------NO PROMOTION 0.00TV 1100.00TV 1500.00TV 3100.00TV 4800.00TV
 5200.00TV 5700.00TV 7100.00TV 7300.00TV 7700.0010 rows selected

Answer: D