MySQL左加入不产生预期结果

Question

我知道有很多很好的问题在左回答回答，但我有麻烦找到有关我的特定问题的东西。我为可能重复的话题表示歉意，但不胜感激。

我有两个包含礼品卡销售的表。我必须验证金额是否匹配。 我正在寻找帮助编写一个查询，从表一回一切即使是表B.不匹配的记录

表A

| id | business_date | am_pm | gift_cards_sold | 
================================================ 
103 | 2011-10-06 | PM | 175.03 
104 | 2011-10-06 | PM | 135.03 
105 | 2011-10-06 | PM | 250.74 
106 | 2011-10-06 | PM | 180.44 
107 | 2011-10-06 | PM | 150.10 
108 | 2011-10-06 | PM | 130.00 

表B

| id | business_date | am_pm | gift_cards_sold | 
================================================ 
103 | 2011-10-06 | PM | 100.03 
105 | 2011-10-06 | PM | 250.74 
106 | 2011-10-06 | PM | 180.44 
107 | 2011-10-06 | PM | 150.10 

这是我的查询到目前为止（显然有什么问题）

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD 
FROM A 
LEFT JOIN B 
USING (id) 
WHERE A.am_pm = 'PM' 
AND A.business_date = '2011-10-06' 
AND B.business_date = '2011-10-06' 
GROUP BY A.id 
ORDER BY A.id ASC 

下面是结果：

| id | A_SOLD | B_SOLD | 
======================== 
103 | 175.03 | 100.03 
105 | 250.74 | 250.74 
106 | 180.44 | 180.44 
107 | 150.10 | 150.10 

正如你所看到的，是用103号方差不过，我需要的结果显示每一个ID，无论是否有匹配与否。结果集是我期望使用Inner Join的结果。

Answer 1

只需从WHERE移动B.business_date = '2011-10-06'条件的ON条款。当您有LEFT联接时，关于第二个表格列（除IS (NOT) NULL之外）的WHERE条件实际上取消LEFT JOIN，则它将作为INNER JOIN。

SELECT A.id AS ID 
    , A.gift_cards_sold AS A_SOLD 
    , B.gift_cards_sold AS B_SOLD 
FROM A 
    LEFT JOIN B 
    ON B.id = A.id 
    AND B.business_date = '2011-10-06' 
WHERE A.am_pm = 'PM' 
    AND A.business_date = '2011-10-06' 
ORDER BY A.id 

Answer 2

你应该使用这样的事情：你为什么用

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD 
FROM A 
LEFT JOIN B 
USING (id) 
WHERE A.am_pm = 'PM' 
AND A.business_date = '2011-10-06' 
AND (B.business_date = '2011-10-06' or B.business_date is null) 
GROUP BY A.id 
ORDER BY A.id ASC 

Answer 3

问题是与B.business_date WHERE条件，也应该允许空

SELECT A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD 
FROM A LEFT JOIN B ON A.id = B.id 
WHERE A.am_pm = 'PM' 
AND A.business_date = '2011-10-06' 
AND (B.business_date = '2011-10-06' OR B.business_date is null) 
ORDER BY A.id ASC 

Answer 4

我不知道一个“group by”子句 - 这里不需要，而在大多数其他数据库中，这会是一个语法错误。

我假设你只需要匹配id上的行。

这应该工作：

select A.id AS ID, A.gift_cards_sold AS A_SOLD, B.gift_cards_sold AS B_SOLD 
from A left join b on b.id = a.id 
where A.business_date = '2011-10-06' 
    and A.am_pm = 'PM' 
order by a.id