mysql - 选择 MySQL Month() 作为空列

Question

有个问题想请教。当我使用左联接时，即使某些行中的值为空值，如何列出所有左表以显示数据？

这是我的查询:

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，查询工作正常，接受它显示所有数据，而不仅仅是 1 个月的交易。

帮忙??请？？知道怎么做吗？

Answer 1

您的 where 子句将您的左联接 转换为内联接，因为您根据联接表的条件过滤数据。

而是将该条件放入联接的 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`;