mysql - 是否有一个子查询可以使用同一表中不同列的最大日期来计算 datediff？

Question

我正在尝试创建一个 MySQL 查询，该查询将查看一列，识别该列中的最新日期，然后在日期差计算中使用该日期来创建新列。

我已经尝试使用 WHERE 语句创建一个新的 maxdate，而不是在 datediff 计算中使用 Now()，但这只是给了我一个包含表中最旧记录的记录，这是差异最大的 datediff 。

SELECT 
    table.firstname
    table.first_order_day
    table.last_order_day
    DATEDIFF(MAX(last_order_day),first_order_day) AS "Customer Tenure"
    DATEDIFF(MAX(last_order_day),last_order_day) AS "Days Since Last Order"
FROM table

预期/期望的结果(MaxDate取自last_order_day列:2019-09-01):

first name,first_order_day,last_order_day,Customer Tenure,Days Since Last Order

Andrew,2019-01-01, 2019-05-01, 243, 123
Audrey,2019-02-01, 2019-09-01, 212, 0
Matt,2019-03-01, 2019-06-01, 184, 92

Actual output:
Audrey,2019-02-01, 2019-09-01, 212, 0

Answer 1

您可以JOIN计算最大值的子查询:

SELECT 
    t.firstname
    t.first_order_day
    t.last_order_day
    DATEDIFF(x.last_order_day, t.first_order_day) AS "Customer Tenure"
    DATEDIFF(x.last_order_day, t.last_order_day) AS "Days Since Last Order"
FROM 
    mytable t
    CROSS JOIN (SELECT MAX(t.last_order_day) last_order_day FROM mytable) x

或者，如果您运行 MySQL 8.0，您也可以使用窗口函数:

SELECT 
    firstname
    first_order_day
    last_order_day
    DATEDIFF(MAX(last_order_day) OVER(), first_order_day) AS "Customer Tenure"
    DATEDIFF(MAX(last_order_day) OVER(), last_order_day) AS "Days Since Last Order"
FROM 
    mytable