/*
  下面是问题中的SELECT语句
*/
-- 确认视图内容
SELECT * FROM ViewPractice5_1;


/*
  下面是解答示例
*/
-- 创建视图的语句
CREATE VIEW ViewPractice5_1 AS
SELECT product_name, sale_price, regist_date
  FROM Product
 WHERE sale_price >= 1000
   AND regist_date = ‘2009-09-20‘;

/*
  下面是问题中的SELECT语句
*/
-- 向视图中添加1行记录
INSERT INTO ViewPractice5_1 VALUES (‘刀‘, 300, ‘2009-11-02‘);


-- 实际上和下面的INSERT语句相同
INSERT INTO Product (product_id, product_name, product_type, sale_price, purchase_price, regist_date) 
            VALUES (NULL, ‘刀‘, NULL, 300, NULL, ‘2009-11-02‘);



/*
  使用PostgreSQL时，需要在INSERT之前
  执行如下语句将视图设定为可以更新
*/
CREATE OR REPLACE RULE insert_rule5_1
AS ON INSERT
TO ViewPractice5_1 DO INSTEAD
INSERT INTO Product (product_name, sale_price, regist_date)
VALUES (new.product_name, new.sale_price, new.regist_date);


/* 
  进行上述设定之后再次执行INSERT时会像下面这样由于NOT NULL约束而发生错误
postgres=# INSERT INTO ViewPractice5_1 VALUES (‘刀‘, 300, ‘2009-11-02‘);
ERROR:  null value in column “product_id" violates not-null constraint
*/

SELECT product_id,
       product_name,
       product_type,
       sale_price,
       (SELECT AVG(sale_price) FROM Product) AS sale_price_all
  FROM Product;

-- 创建视图的语句
CREATE VIEW AvgPriceByType AS
SELECT product_id,
       product_name,
       product_type,
       sale_price,
       (SELECT AVG(sale_price)
          FROM Product P2
         WHERE P1.product_type = P2.product_type
         GROUP BY P1.product_type) AS avg_sale_price
 FROM Product P1;

-- 确认视图内容
SELECT * FROM AvgPriceByType;