以下的文章主要講述的是MySQL關聯left join 條件on與where 條件的不同之處，我們現在有兩個表，即商品表(products)與sales_detail(銷售記錄表)。我們主要是通過這兩個表來對MySQL關聯left join 條件on與where 條件的不同之處進行講述。

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 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）。

MySQL關聯left join條件on和where條件的區別表的結構 `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;

導入表中的數據 `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; 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');

MySQL關聯left join條件on和where條件的區別區別兩條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 | +-------+-------+------------+ | 商品1 | AC90 | 2008-09-23 | +-------+-------+------------+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/moqiang02/p/4061165.html

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