为空列选择 MySQL Month()

Question

有问题要问。当我使用左连接时，即使某些行中有空值，如何列出所有左表以显示数据？

这是我的问题：

SELECT *, SUM(transactions.debit_credit) current_spending 
FROM (`budget`) LEFT JOIN `categories` ON `budget`.`category`=`categories`.`idcategory` 
LEFT JOIN `user_category` ON `budget`.`category`=`user_category`.`idcategory` 
LEFT JOIN `category_transaction` ON `budget`.`category`=`category_transaction`.`idcategory` 
LEFT JOIN `transactions` ON `category_transaction`.`idtransaction`=`transactions`.`idtransaction` 
WHERE `user_category`.`iduser` = '1' 
AND MONTH(transactions.transaction_date) = '11' 
GROUP BY `budget`.`idbudget`;

当我删除 AND MONTH(transactions.transaction_date) = '11' 时，查询可以正常工作，接受它会显示所有数据，而不是仅显示 1 个月的交易。

帮助？？请？？知道怎么做吗？？

Answer 1

您的where 子句将您的left join 转换为inner join，因为您根据联接表的条件过滤数据。

而是将该条件放在您的联接的 on 子句中

SELECT *, SUM(transactions.debit_credit) current_spending 
FROM (`budget`) 
LEFT JOIN `categories` ON `budget`.`category`=`categories`.`idcategory` 
LEFT JOIN `user_category` ON `budget`.`category`=`user_category`.`idcategory` 
                         AND `user_category`.`iduser` = '1' 
LEFT JOIN `category_transaction` ON `budget`.`category`=`category_transaction`.`idcategory` 
LEFT JOIN `transactions` ON `category_transaction`.`idtransaction`=`transactions`.`idtransaction` 
                        AND MONTH(transactions.transaction_date) = '11' 
GROUP BY `budget`.`idbudget`;