How to find number of products having SALE price 50% of MRP using MYSQL?
In order to (finally) solve your problem, I did the following

CREATE TABLE product
(
  id INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR (15) NOT NULL,
  price_type VARCHAR (10) NOT NULL,
  price INTEGER NOT NULL
);


Populate the table - I've added extra records, because, as I've found (again), testing with 1 record (or set - i.e. name = 'book') - I added books 1, 2 and 3, because I doubt if you're only selling one book and pens 1 and 2.

INSERT INTO product (name, price_type, price)
VALUES 
('book1', 'MRP',  10),
('book1', 'SALE',  5),
('pen1', 'MRP',  10),
('pen1', 'MRP',  5),

('book2', 'MRP',  14),
('book2', 'SALE',  7),
('pen', 'MRP',  16),

('book3', 'MRP',  30),
('book3', 'SALE',  15),
('pen', 'MRP',  8);


The query

SELECT SUM(cnt) AS number_of_50
FROM
(
  SELECT  p1.name, COUNT(p1.name) AS cnt
  FROM product p1
  JOIN product p2
    ON p1.name = p2.name
  WHERE p1.price = 2 * p2.price
  GROUP BY p1.name
) AS tab


Result

number_of_50
5


dbfiddle here!

p.s. Apart from having more than one record, you might consider using a fiddle for your tables and data for any questions in the future? Also, please also always supply your version of MySQL, it can make a real difference to the answer. There are some articles on how to ask questions here on my profile - you might want to take a look? Welcome to the forum!