Question

我正在使用一个简单的存储过程从两个表中收集数据并发送回PHP：

BEGIN

DECLARE cur_name VARCHAR(50);

SELECT currency_name INTO cur_name FROM currencies WHERE id = currency_id_var;
SELECT cur_name, sum(amount) AS income_amount, currency_id, month(income_date) AS month_of_income 
    FROM incomes 
    WHERE user_id = user_id_var 
        AND currency_id = currency_id_var 
        AND income_date > year_beginning_var 
        AND month(income_date) = month_var 
    GROUP BY currency_id;

END

它的工作正常，除非在'收入'表中找不到记录。我需要来自程序的响应，其中'cur_name'设置（它总是有一些值），其余变量（income_amount，currency_id，month_of_income）设置为NULL。我问的是一些无法做到的事情吗？我试图使它与 SELECT IFNULL 和 UNION SELECT NULL 类命令一起工作，但没有成功...... 谢谢！

Answer 1

您可以将两个查询合并到LEFT JOIN中，如下所示：

SELECT currency_name as cur_name, 
       income_amount,
       currency_id,
       month_of_income
    FROM currencies
    LEFT JOIN
        (
            SELECT cur_name, sum(amount) AS income_amount, currency_id, month(income_date) AS month_of_income 
                FROM incomes 
                WHERE user_id = user_id_var 
                    AND currency_id = currency_id_var 
                    AND income_date > year_beginning_var 
                    AND month(income_date) = month_var 
                GROUP BY currency_id;
        )
        ON currency_id = currencies.id
    WHERE currencies.id = currency_id_var;

未经测试，但您明白了：它是LEFT JOIN，因此保证为左表（currencies）中的每个匹配行返回至少一行，如果是右表（子查询）不会产生任何结果，相应的字段将返回为NULL。

为了更好的可读性，我会考虑将子查询移动到视图，然后左键连接。

如果没有记录，Mysql如何返回NULL值

1 个答案: