IFNULL() MySQL 与 ROLLUP 结合使用时无法按预期工作答案

【问题标题】：IFNULL() MySQL not working as expected in combination with ROLLUPIFNULL() MySQL 与 ROLLUP 结合使用时无法按预期工作
【发布时间】：2021-10-02 10:44:12
【问题描述】：

我有一个 MySQL 查询。

SELECT 
YEAR(Date) 'Year',
MONTH(Date)'Month',
SUM(Amount)'Amount' 
FROM Bill.Statistics 
LEFT JOIN Employees.Employee
ON Bill.Statistics.Employee = Employees.Employee.ID
WHERE Employees.Employee.Department LIKE '%%' 
AND Bill.Statistics.Employee LIKE '%%'
AND Date>='2020-10-01' AND Date<='2021-03-31' 
GROUP BY YEAR(Date),MONTH(Date) 
WITH ROLLUP

这给了我一个结果。

Year	Month	Amount
2020	10	10.10
2020	11	11.11
2020	12	12.12
2020	NULL	33.33
2021	1	1.01
2021	2	2.02
2021	3	3.03
2021	NULL	6.06
NULL	NULL	39.39

现在，当我尝试使用 IFNULL() 来摆脱 NULL 并在此处放一个破折号时，我会通过以下代码得到以下结果。

SELECT 
IFNULL(YEAR(Date), '-') AS 'Year',
IFNULL(MONTH(Date),'-') AS 'Month',
SUM(Amount)'Amount' 
FROM Bill.Statistics 
LEFT JOIN Employees.Employee
ON Bill.Statistics.Employee = Employees.Employee.ID
WHERE Employees.Employee.Department LIKE '%%' 
AND Bill.Statistics.Employee LIKE '%%'
AND Date>='2020-10-01' AND Date<='2021-03-31' 
GROUP BY YEAR(Date),MONTH(Date) 
WITH ROLLUP

Year	Month	Amount
2020	10	10.10
2020	11	11.11
2020	12	12.12
2020	12	33.33
2021	1	1.01
2021	2	2.02
2021	3	3.03
2021	3	6.06
2021	3	39.39

就像你一样，它只是将前一个值的最后一个值复制到 NULL 所在的单元格中。

谁能告诉我为什么会这样？我只是想让所有的 NULL 都变成简单的破折号。

【问题讨论】：

标签： mysql

【解决方案1】：

据我所知，汇总行（生成那些 NULL 值according to docs）是在选择数据“之后”生成的。我认为您应该像这样包装 SELECT：

SELECT
    IFNULL(s.`Year`, 'Total Year') AS `Year`,
    IFNULL(s.`Month`, 'Total Month') AS `Month`,
    s.`Amount`
FROM

(SELECT 
    YEAR(Date) 'Year',
    MONTH(Date) 'Month',
    SUM(Amount) 'Amount' 
FROM Bill.Statistics 
    LEFT JOIN Employees.Employee
    ON Bill.Statistics.Employee = Employees.Employee.ID
WHERE Employees.Employee.Department LIKE '%%' 
    AND Bill.Statistics.Employee LIKE '%%'
    AND Date>='2020-10-01' AND Date<='2021-03-31' 
    GROUP BY YEAR(Date),MONTH(Date) 
    WITH ROLLUP
) AS s

【讨论】：

这正是发生的事情！现在我明白为什么会这样了。感谢您的快速回复。