如何在现有列中添加值？ （MYSQL）

Question

我有3列:( ToDate，FromDate，Diff）。

我想自动为Diff列添加值，ToDate和FromDate之间存在差异，并希望将其保存到表格中。

我可以使用

手动创建临时Diff列

SELECT ToDate,FromDate, (TO_DAYS(FromDate) - TO_DAYS(ToDate)) AS Diff FROM tbl_name，

但它不会保存到Diff列

所以我最终输入了这个：

INSERT INTO tbl_name (Diff) SELECT (TO_DAYS(FromDate) - TO_DAYS(ToDate)) AS Diff FROM tbl_name

我想要这样的事情：

FromDate  
2018-05-22

ToDate    
2018-05-23

Diff

1