text: Create table If Not Exists Product (product_id int, product_name varchar(10), unit_price int);
  Create table If Not Exists Sales (seller_id int, product_id int, buyer_id int, sale_date date, quantity int, price int);
  Truncate table Product;
  insert into Product (product_id, product_name, unit_price) values ('1', 'S8', '1000');
  insert into Product (product_id, product_name, unit_price) values ('2', 'G4', '800');
  insert into Product (product_id, product_name, unit_price) values ('3', 'iPhone', '1400');
  Truncate table Sales;
  insert into Sales (seller_id, product_id, buyer_id, sale_date, quantity, price) values ('1', '1', '1', '2019-01-21', '2', '2000');
  insert into Sales (seller_id, product_id, buyer_id, sale_date, quantity, price) values ('1', '2', '2', '2019-02-17', '1', '800');
  insert into Sales (seller_id, product_id, buyer_id, sale_date, quantity, price) values ('2', '1', '3', '2019-06-02', '1', '800');
  insert into Sales (seller_id, product_id, buyer_id, sale_date, quantity, price) values ('3', '3', '3', '2019-05-13', '2', '2800');
  