Mysql查询三个表

Question

我试图编写一个连接三个表的查询。 正如您在下图所示，我有：

• 用户表
• 费用表
• 费用类别表

我需要获得一个结果表，按类别类型分类费用（图像上的结果表）。

select users.email,
(
    SELECT IFNULL(sum(expenses.amount),0)
    FROM expenses
    JOIN expense_categories as cat
    ON cat.id  = expenses.category_id
    AND cat.type = 'home'
    WHERE expenses.user_id = users.id
) as 'home expenses'
from users

此查询可以正常运行，但我确定不是正确的方法。

你能帮帮我吗？

非常感谢

Answer 1

您想要的查询有join个和一个聚合：

SELECT u.id, u.email,
       SUM(CASE WHEN ec.type = 'home' THEN e.amount ELSE 0 END) as home, 
       SUM(CASE WHEN ec.type = 'job' THEN e.amount ELSE 0 END) as job 
FROM users u LEFT JOIN
     expenses e
     ON u.id = e.user_id LEFT JOIN
     expense_categories ec
     ON ec.id  = e.category_id
GROUP BY u.id, u.email;