mysql - 在表 SQL 中生成日期

Question

我有一个 SQL 数据库，其中包含有关学生和他们借阅的书籍的信息。

我有一张外借表，其中包括book_id、student_id、借书日期和可以保留的天数。

我想通过添加这本书拍摄日期的天数来自动生成 return_date。我如何使用 MySQL Workbench 执行此操作？

Answer 1

您可以使用 MySQL datetime function date_add()计算目标 return_date。假设借书的日期叫做loan_date，loan_duration中保存的是借书时长，单位为天:

select
    book_id,
    student_id,
    loan_date,
    date_add(loan_date, interval loan_duration day) return_date
from loan_table

在更新语句中:

update loan_table
set return_date = date_add(loan_date, interval loan_duration day)
where return_date is null -- only compute the return_dates that were not yet computed

---

编辑

如果您希望该值自动生成，一个选项是使用生成的存储(或虚拟)列(自 MySQL 5.7 起可用)。这通过定义一个计算规则来实现，MySQL 将在插入或更新记录时自动应用该规则。

Demo on DB Fiddle :

-- set up
create table loan_table(
    id int primary key auto_increment,
    book_id int,
    student_id int,
    loan_date date,
    loan_duration int,
    -- here is the generated column
    return_date date 
        as (date_add(loan_date, interval loan_duration day)) stored
);

-- perform an insert
insert into loan_table(book_id, student_id, loan_date, loan_duration)
values(1, 1, '2019-11-02', 3)

-- check the computed value
select * from loan_table;

id | book_id | student_id | loan_date  | loan_duration | return_date
-: | ------: | ---------: | :--------- | ------------: | :----------
 1 |       1 |          1 | 2019-11-02 |             3 | 2019-11-05