148. View the Exhibit and examine the structure of the PRODUCTS tables.
You want to generate a report that displays the average list price of product categories where the average
list price is less than half the maximum in each category.
Which query would give the correct output?
A. SELECT prod_category,avg(prod_list_price)
FROM products
GROUP BY prod_category
HAVING avg(prod_list_price) < ALL 小于所有，小于最小值
(SELECT max(prod_list_price)/2
FROM products
GROUP BY prod_category);
B. SELECT prod_category,avg(prod_list_price)
FROM products
GROUP BY prod_category
HAVING avg(prod_list_price) > ANY
(SELECT max(prod_list_price)/2
FROM products
GROUP BY prod_category);
C. SELECT prod_category,avg(prod_list_price)
FROM products
HAVING avg(prod_list_price) < ALL
(SELECT max(prod_list_price)/2
FROM products
GROUP BY prod_category);
D. SELECT prod_category,avg(prod_list_price)
FROM products
GROUP BY prod_category
HAVING avg(prod_list_price) > ANY
(SELECT max(prod_list_price)/2
FROM products);
Answer: A
答案解析：
1、首先查询子查询中出每个PROD_CATEGORY的prod_list_price的最大值的1/2的值。
sh@TEST0924> SELECT max(prod_list_price)/2 FROM products
2 GROUP BY prod_category;
MAX(PROD_LIST_PRICE)/2
----------------------
99.995
649.995
299.995
549.995
499.995
2、找出prod_list_price的平均值
sh@TEST0924> SELECT prod_category,avg(prod_list_price)
2 FROM products
3 GROUP BY prod_category;
PROD_CATEGORY AVG(PROD_LIST_PRICE)
-------------------------------------------------- --------------------
Software/Other 31.4130769
Hardware 1149.99
Electronics 121.682308
Photo 230.19
Peripherals and Accessories 145.085238
3、平均值小于1/2最大值的所有值 ，即小于最小值
sh@TEST0924> SELECT prod_category,avg(prod_list_price) FROM products GROUP BY prod_category
2 having avg(prod_list_price)<all(SELECT max(prod_list_price)/2 FROM products GROUP BY prod_category);
PROD_CATEGORY AVG(PROD_LIST_PRICE)
-------------------------------------------------- --------------------
Software/Other 31.4130769
故选A答案。
B与题意不符，> ANY大于最小值
sh@TEST0924> SELECT prod_category,avg(prod_list_price)
2 FROM products
3 GROUP BY prod_category
4 HAVING avg(prod_list_price) > ANY
5 (SELECT max(prod_list_price)/2
6 FROM products
7 GROUP BY prod_category);
PROD_CATEGORY AVG(PROD_LIST_PRICE)
-------------------------------------------------- --------------------
Hardware 1149.99
Electronics 121.682308
Photo 230.19
Peripherals and Accessories 145.085238
C语法错误，缺少group by
sh@TEST0924> SELECT prod_category,avg(prod_list_price)
2 FROM products
3 HAVING avg(prod_list_price) < ALL
4 (SELECT max(prod_list_price)/2
5 FROM products
6 GROUP BY prod_category);
SELECT prod_category,avg(prod_list_price)
*
ERROR at line 1:
ORA-00937: not a single-group group function
D与题意不符，没有in each category分组。
sh@TEST0924> SELECT max(prod_list_price)/2 FROM products;
MAX(PROD_LIST_PRICE)/2
----------------------
649.995
sh@TEST0924> SELECT prod_category,avg(prod_list_price)
2 FROM products
3 GROUP BY prod_category
4 HAVING avg(prod_list_price) > ANY
5 (SELECT max(prod_list_price)/2
6 FROM products);
PROD_CATEGORY AVG(PROD_LIST_PRICE)
-------------------------------------------------- --------------------
Hardware 1149.99