第三章 复杂一点的查询

Q1

CREATE VIEW ViewPractice5_1(product_type, sale_price, regist_date)
AS
SELECT product_type, sale_price,regist_date
  FROM product  
 WHERE regist_date = "2009-09-20" and  sale_price>=1000;

Q2

SQL 错误 [1423] [HY000]: Field of view 'shop.viewpractice5_1' underlying table doesn't have a default value

还是因为视图的定义，视图只是原表的一个窗口，所以它修改也只能修改透过窗口能看到的内容。

Q3

SELECT product_id,
       product_name,
       sale_price,
       (SELECT AVG(sale_price)
          FROM product) AS avg_price
  FROM product;

Q4

SELECT product_id, product_name, product_type, sale_price,
 (SELECT AVG(sale_price)
    FROM product AS p2
  WHERE p1.product_type=p2.product_type
  GROUP BY product_type) AS sale_price_all
FROM product AS P1

Q5

运算或者函数中含有 NULL 时，结果全都会变为NULL .

Q6

运行结果1：返回purchase_price结果不为500，2800，5000，且不为NULL 的行。
运行结果2：返回为空。就是上题提到的运算或函数中含有 NULL 时，结果全都会变为NULL 。筛选含/不含NULL的项，用IS NULL或IS NOT NULL.

Q7

SELECT
SUM(CASE WHEN sale_price BETWEEN 0 AND 1000 THEN 1 ELSE 0 END) AS low_price,
SUM(CASE WHEN sale_price BETWEEN 1001  AND 3000 THEN 1 ELSE 0 END) AS mid_price,
SUM(CASE WHEN sale_price>3000 THEN 1 ELSE 0 END) AS high_price
  FROM product;