Chapter 2 basic query and sorting

Q2.1

SELECT product_name, regist_date
  FROM product
 WHERE regist_date >'2009-04-28';

Q2.2

product_id|product_name|product_type|sale_price|purchase_price|regist_date|
----------+------------+------------+----------+--------------+-----------+

Q2.3

SELECT product_name, sale_price, purchase_price
  FROM product
 WHERE sale_price - purchase_price>=500;
 
SELECT product_name, sale_price, purchase_price
  FROM product
 WHERE  purchase_price-sale_price <=-500;

Q2.4

SELECT product_name,product_type, sale_price*0.9-purchase_price   As "profit"
  FROM product
 WHERE  sale_price*0.9-purchase_price >100;

Q2.5

The errors that have occurred are ：

1. sum Functions can only use numeric columns as parameters
2. At the end of the aggregate function SELECT Clause specifies the use of columns other than aggregate keys COUNT Wait for the aggregate function ,SELECT If the column name appears in the clause , Can only be GROUP BY The column name specified in clause （ That's the aggregate bond ）.
3. WHERE The clause is written in GROUP BY After Clause

Q2.6

SELECT product_type,SUM(sale_price), SUM(purchase_price) 
 FROM product
GROUP BY product_type
HAVING SUM(sale_price)>1.5*SUM(purchase_price) ;

Q2.7

select * from product 
order by - regist_date;