Question

我正在尝试只在我加入的表中的一列= 1时才进行LEFT JOIN。

这是我到目前为止所拥有的。

SELECT * FROM size, oil LEFT JOIN purchase ON oil. oilId = purchase. purOilId
WHERE sizId = oilSizeId AND oilUnused ='' ORDER BY oilOurName

我需要添加的内容如下。

LEFT JOIN purhcase (only if purMark = 1)

这是我不知道该怎么办。

以后使用AND是行不通的，因为油表需要定义结果中有多少行。

由于

Answer 1

将AND条件添加到JOIN，如下所示

SELECT * FROM size, oil LEFT JOIN purchase ON oil.oilId = purchase.purOilId AND purchase.purMark = 1  
WHERE sizId = oilSizeId AND oilUnused ='' ORDER BY oilOurName

Answer 2

在mysql中，我认为你能做到这一点的唯一方法是在你的连接中使用一个子查询，类似于：

SELECT * FROM size, oil 
LEFT JOIN (
   SELECT * 
   FROM purchase
   WHERE purMark = 1
) purchase ON purchase.purOilId = oil.oilId /* or whatever your joining criteria is */
WHERE sizId = oilSizeId AND oilUnused ='' 
ORDER BY oilOurName

希望这有帮助。

Answer 3

尝试以下：

SELECT * FROM size, oil 
LEFT JOIN purchase ON oil. oilId = purchase. purOilId
WHERE purMark = 1
sizId = oilSizeId AND oilUnused ='' 
UNION
SELECT * FROM size, oil 
WHERE purMark != 1
sizId = oilSizeId AND oilUnused =''

或者

SELECT * FROM size, oil 
LEFT JOIN purchase ON oil. oilId = purchase. purOilId AND purMark = 1
WHERE sizId = oilSizeId AND oilUnused =''

Answer 4

奇怪的是，比这里晚了一周，here提出并回答了这个问题

mySQL LEFT JOIN仅限于

4 个答案: