尝试根据 mySQL 中 SELECT 语句的结果更改表

Question

when i use the following select statement i can return the date differences between date_due and date_returned.

select Date_Due,Date_Returned,
datediff(Date_Due,Date_Returned) as Day_Diff 
From tp_rental

However i cant get these values to replace the current null values in the tp_rental table. I tried the below statement also thinking it would create a new column.

alter table tp_rental
ADD DayDiff as DATEDIFF (Date_Due,Date_Returned);

my errors say there is a problem with the syntax near as DATEDIFF

any ideas? thanks in advance

Answer 1

Generated column expressions need to be enclosed in () (see the manual ). You also need to add a column type definition:

alter table tp_rental
ADD DayDiff INT as (DATEDIFF(Date_Due,Date_Returned));