Additional data added, MySQL

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#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\\' is a delimiter

-- select version() as 'mysql version';

drop table if exists transactions;

CREATE TABLE transactions(id INT, transaction_time DATE, item VARCHAR(20), unit_price DOUBLE, quantity INT,  subtotal DOUBLE);
INSERT INTO transactions VALUES
(1,      '2012-5-15',          'A',         1.00,         15,         15.00) ,
(2,      '2012-5-15',          'A',         3.00,         15,         45.00),
(3,      '2012-5-15',          'B',         1.00,         10,         10.00),
(4,      '2012-6-10',         'A',        2.00,         15,         30.00),
(5,      '2012-6-15',          'A',         2.00,         10,         20.00),
(6,      '2012-6-16',          'A',         1.00,         2,          10.00);

SELECT tr1.transaction_time, tr1.item, 
     IF (tr1.unit_price=tr2.unit_price, tr1.quantity, tr2.quantity) as t_q,  
     IF (tr1.unit_price=tr2.unit_price, tr1.unit_price, tr2.unit_price) as t2_q 
     FROM transactions tr1 LEFT JOIN transactions tr2 
     ON 
     tr1.transaction_time>=tr2.transaction_time AND tr1.item=tr2.item
GROUP BY tr1.item, tr1.transaction_time, t2_q      
order by tr1.item, tr1.transaction_time


absolute service time: 0,35 sec