現有兩個表，商品表(products)和sales_detail(銷售記錄表)。如下圖:

products:
????? pid pname pcode
??? 1?? 商品1?? AC90
??? 2?? 商品2 DE78
???? 3?? 商品3??? XXXX

sales_detail:
??? aid pcode saletime
??? 1?? AC90 2008-09-22
??? 2?? DE78 2008-09-22
??? 3?? AC90 2008-09-23
??? 4?? AC90 2008-09-24

按商品在某個時間段內的銷售量來排行，比如我想統計23－24號這兩天的銷售數量并排行。（注：DE78這個商品在這兩天沒有銷售，但是也要顯示出來，只是數量為0）。


-- 表的結構 `products`
--
CREATE TABLE `products` (
?? `pid` int(3) NOT NULL auto_increment,
?? `pname` varchar(20) NOT NULL,
?? `pcode` varchar(20) NOT NULL,
?? PRIMARY KEY?? (`pid`)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
--
-- 導出表中的數據 `products`
--
INSERT INTO `products` (`pid`, `pname`, `pcode`) VALUES (1, '商品1', 'AC90'),
(2, '商品2', 'DE78'),
(3, '商品3'. 'XXXX');
-- --------------------------------------------------------
--
-- 表的結構 `sales_detail`
--
CREATE TABLE `sales_detail` (
?? `aid` int(3) NOT NULL auto_increment,
?? `pcode` varchar(20) NOT NULL,
?? `saletime` date NOT NULL,
?? PRIMARY KEY?? (`aid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1;
--
-- 導出表中的數據 `sales_detail`
--
INSERT INTO `sales_detail` (`aid`, `pcode`, `saletime`) VALUES (1, 'AC90', '2008-09-22'),
(2, 'DE78', '2008-09-22'),
(3, 'AC90', '2008-09-23'),
(4, 'AC90', '2008-09-24');

區別兩條sql語句：

select p.pname,p.pcode,s.saletime,count(s.aid) as total from products as p
left join sales_detail as s on (s.pcode=p.pcode)
where s.saletime in ('2008-09-23','2008-09-24')
group by p.pcode order by total desc,p.pid asc

+---------+-------+------------+-------+
| pname?? | pcode | saletime?? | total |
+---------+-------+------------+-------+
| 商品1?? | AC90 | 2008-09-23 |???? 2 |
+---------+-------+------------+-------+

select p.pname,p.pcode,s.saletime,count(s.aid) as total from products as p
left join sales_detail as s on ((s.pcode=p.pcode) and s.saletime in ('2008-09-23','2008-09-24'))
group by p.pcode order by total desc,p.pid asc

+---------+-------+------------+---------+
| pname?? | pcode | saletime?? | total |
+---------+-------+------------+-------+
| 商品1?? | AC90 | 2008-09-23 |???? 2 |
| 商品2?? | DE78 | NULL?????? |???? 0 |
| 商品3?? | XXXX?? | NULL?????? |???? 0 |
+---------+-------+------------+---------+

心得：on中的條件關聯，一表數據不滿足條件時會顯示空值。where則輸出兩表完全滿足條件數據。

轉載于:https://www.cnblogs.com/flyfesh/archive/2009/07/03/1516507.html

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