Why am I getting wrong values from my joined tables? - php

Question

Here is my MySQL schema
CREATE TABLE IF NOT EXISTS `sales` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`date` date NOT NULL,
`total_tax` decimal(25,2) NOT NULL,
`total` decimal(25,2) NOT NULL,
`total_tax2` decimal(25,2) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=38 ;
INSERT INTO `sales` (`id`, `date`, `total_tax`, `total`, `total_tax2`) VALUES
(1, '2013-02-14', 6, 100, 21),
(2, '2013-02-18', 6, 100, 21),
(3, '2013-03-01', 6, 100, 21),
(4, '2013-03-07', 6, 100, 21),
(5, '2013-03-28', 6, 100, 21),
(6, '2013-03-28', 6, 100, 21),
(7, '2013-04-04', 6, 100, 21);
CREATE TABLE IF NOT EXISTS `purchases` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`date` date NOT NULL,
`total` decimal(25,2) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=15 ;
INSERT INTO `purchases` (`id`, `date`, `total`) VALUES
(1, '2013-02-15', 150),
(2, '2013-02-16', 18),
(3, '2013-03-05', 80),
(4, '2013-03-09', 50),
(5, '2013-03-16', 500),
(6, '2013-03-22', 200);
And I am trying to get the total values of sales.total, total_tax, total_tax2 and purchases.total by month by joining table on date. I am trying this SQL query:
SELECT date_format( sales.date, '%b %Y' ) AS MONTH,
SUM( COALESCE( sales.total, 0 ) ) AS sales,
SUM( COALESCE( purchases.total, 0 ) ) AS purchases,
SUM( COALESCE( total_tax, 0 ) ) AS tax1,
SUM( COALESCE( sales.total_tax2, 0 ) ) AS tax2
FROM sales
LEFT JOIN purchases
ON date_format(purchases.date, '%b %Y' ) = date_format(sales.date, '%b %Y' )
WHERE sales.date >= date_sub( now( ) , INTERVAL 12 MONTH )
GROUP BY date_format( purchases.date, '%b %Y' )
ORDER BY date_format( sales.date, '%m' ) ASC
and getting results as
Feb 2013 400 336 24 84
Mar 2013 1600 3320 96 336
Apr 2013 100 0 6 21
Why am I getting these incorrect values?
SQL Fiddle

Answer 1

You are joining by the month of the dates of your tables, meaning that you are multiplying the rows that you are suming. You either perform the aggregation previously or join by the key of your tables:
SELECT S.Month,
S.sales,
ISNULL(P.purchases,0) purchases,
S.tax1,
S.tax2
FROM ( SELECT date_format(date, '%b %Y') Month,
SUM(total) Sales,
SUM(total_tax) tax1,
SUM(total_tax2) tax2
FROM sales
WHERE sales.date >= date_sub( now( ) , INTERVAL 12 MONTH )
GROUP BY date_format(date, '%b %Y')) S
LEFT JOIN ( SELECT date_format(date, '%b %Y') Month,
SUM(total) purchases
FROM purchases
GROUP BY date_format(date, '%b %Y')) P
ON S.Month = P.Month
GROUP BY S.Month
ORDER BY S.Month
Here is the sqlfiddle with this option.