Description

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The store you’re working for has decided to organize a special holiday event: every 4th purchase made during this event grants the lucky person who made it a special prize.

All purchases made during the event are stored in the database table purchases with the following structure:

• timestamp: unique purchase timestamp;
• buyer_name: the name of the person who made this purchase.

Given the purchases table, compose the resulting table with one column winners containing the names of the buyers who won the special prize by making a purchase number k \* 4 for some integer k. The numbering of the purchases starts with 1.

Note, that each person can get no more than one prize (i.e. their name can occur in the answer at most once).

The table should be sorted by the winners’ names in ascending order.

Example

For the following tables purchases

timestamp	buyer_name
2014-11-09 15:23:05	Frank West
2014-11-09 20:11:02	Terrence Alexander
2014-11-10 12:10:00	Sandy Cohen
2014-11-10 13:00:11	Frank West
2014-11-10 14:09:10	Sandy Cohen
2014-11-10 14:15:15	Leonard Grant
2014-11-10 17:09:10	Frank West
2014-11-10 19:09:10	Diane Tucker
2014-11-11 18:09:11	Pauline Ross
2014-11-11 20:00:00	Jasmine Gibson
2014-11-12 10:12:00	Kim Neal
2014-11-12 10:12:01	Frank West
2014-11-12 15:14:42	Sean Kim

the output should be

winners
Diane Tucker
Frank West

because Frank West made the 4th and the 12th purchases, and Diane Tucker’s purchase was the 8th.

• [execution time limit] 10 seconds (mysql)

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/*Please add ; after each select statement*/
CREATE PROCEDURE holidayEvent()
BEGIN
    SET @purchase = 0;
	SELECT DISTINCT buyer_name AS winners
    FROM(
        SELECT * FROM purchases
        WHERE (@purchase:=@purchase + 1)%4 = 0
        ORDER BY timestamp) t
    ORDER BY buyer_name;
END